Why is it that the Rishonim and many Achronim used to be fluent in philosophy (mainly Greek) and these days it's unheard of? Is it due to our weakness, and it is really allowed?
Aside from all the other answers, it's worth mentioning that no one studies Greek philosophy today. The thinkers that were so respected by (some of) the Rishonim are mostly irrelevant even to secularists.
Their views on science were, well, wrong. For instance, Aristotle seems to have believed that a heavy object falls faster under gravity than a lighter object (see https://hsm.stackexchange.com/questions/7664/how-did-people-believe-aristotles-law-of-gravity-for-so-long), it took Galileo to do careful experiments to begin getting the rules right. They struggled along with epicycles.
Rejecting many of the things the ancients believed about the infinite was the basis of Cantor's Set Theory. [See, for instance, https://mathworld.wolfram.com/NonstandardAnalysis.html for how this has altered mathematics.]
Coming to a better understanding of how axioms work allowed the development of non-Euclidean geometry, closing an issue (the "Parallel Postulate") that had been a problem since the time of Euclid.
In short, those Rishonim who studied Greek philosophy did so because they wanted to be up-to-date on the science of the day. That isn't true in our time; if you want to be up-to-date there're a whole lot of other things that are more urgent. We are more likely to study it so that we can read the beginning of Chovos Halevovos.
Personal note: Some of us once went to Rav Yaakov Weinberg z"l, Rosh Yeshiva of Ner Yisroel, and asked if he would give a regular chaburah on Moreh Nevuchim. He answered that a lot of the sefer deals with issues (Aristotle's philosophy, the Kalam, ...) that were great concerns in the Rambam's time, but not today.
If it just knowledge it is permitted to be learned (to add to your Torah learning and fear of heaven) even though most of your learning should be in Torah (if you are not learning Torah you should not learn it)
But it is regarding G-d and his connection with the world then
It is and was forbidden, but there are exceptions for example if the philosophy is being used against Jews/Judaism then it needs to be learnt to be able to save Jews/Judaism, but in our generation Jews/Judaism are left alone and respected, there is no need to learn it
The mitzvah of learning Torah (day and night) means you should not be learning other things, but sometimes and a little bit it is permitted for a talmid chachom to learn other knowledge, since he can learn from them Torah and fear of G-d but not other people (only a talmid chochom)
This is only if they are not books of minim which are books of philosophers of the nations of the world which were minim and kofrim of "G-d's care of the world" and of prophecy, since it is forbidden to read and (or) look in them always even to learn from them some musar and fear of G-D, so even when their words are brought in Jewish books, you need to be careful (to stay away) from them, and about them our sages said "the one that reads outside books does not have a portion in the world to come, the only reason some sages delt with them was to answer them and to strengthen our religion, and in those times the time needed it, to answer the minim of the nations of the world, that where in those generations argument with the Jews, but not in out generations.
PS it is possible that some Jewish sages know it but it does not mean it was permitted for them to learn, even if you ask them they might confess that it was wrong
This question asks about shifting attitudes over many centuries, and is somewhat open ended, so I will begin with general historical overview:
The vast majority of philosophy and non-Talmudic study was by the Geonim and their intellectual successors, North African and Southern Spanish rishonim (such as Rabbenu Chanael b. Chushiel of Tunisia, and Rambam of Cordoba.) Mention must also be made of the scholars of Provence in Southern France who absorbed this positive attitude towards philosophy. This breed of rishonim pretty much died out, after Babylonian academies diminished tremendously by the 12th century. That same century saw the al-Mohades invade the South of Spain, effectively ending Jewish cultural life there.
After that (13th-14th century) "Sefardi" Jews continued in Northern (Christian) Spain and were very different from the Sefardim of the past, and were very influenced by the Northern French Tosafists. The leaders of this period were Ramban and R. Yonah of Gerona. Their students; Ra'ah, Rashba, and Ritva were all heavily influenced by the French. The French, like Ashkenazim in general, were generally anti-intellectual and opposed to philosophy in particular, and anything non-Talmudic in general. A case in point is Rashba's (admittedly non-universal) ban on the study of philosophy. This attitude expressed itself not just in the absence of study of philosophy in the strict sense of the word, but also of the study of astronomy, mathematics, and other sciences. It even resulted in decreased study of Scripture among Ashkenzim!
To quote Prof. A.S. Halkin:
The essentially negative attitude toward philosophy, characteristic of the rabbis of the Franco-German tradition, penetrated Provence and Spain during the thirteenth century, together with their method of Talmudic study, which was being adopted, and their views on that study. The role of Rabbi Asher ben Yehiel, a German Tosaphist, who was making his way to Spain at this time to become the Rabbi of Toledo, was significant in galvanizing the energy of the leaders to act, as it was in creating an atmosphere of piety in Spain. His feelings about philosophy were decidedly hostile, and he disliked secular studies, of which he admitted he knew nothing, as intensely as philosophy. (Yedaiah Bedershi's Apology p.183)
At the same time Jewish mysticism developed into what became known as "Kabbalah". From its infancy in 12th century Provence it continued with the publication of the Zohar in Spain at the end of the 13th century. Kabbalah became increasingly popular in the ensuing centuries, especially Lurianic Kabbalah which continued to increase in popularity, peaking in 17th century Poland. (Whereupon interest waned somewhat following the tragedy of Shabbettai Tsevi, a kabbalistic false Messiah, who sparked cults and movements devoted to him that were very influential throughout the 18th century).
However, it would be a mistake to see Kabbalah as the cause for anti-rationalism. This fails to appreciate that the Ashkenazi culture was averse to all non-Talmudic study and was in general superstitious, even before the rise of Kabbalah. More accurately, the rise of Kabbalah was itself a reaction of these Ashkenazi tendencies, although the popularity of Kabbalah certainly succeeded in spreading Ashkenazi anti-rationalism.
In the 14th and 15th centuries, the Arba'ah Turim written by an Ashkenazi immigrant gains great popularity in the increasingly Ashkenazi Spain, until the Spanish expulsion of 1492. In the next couple of centuries Ashkenazi culture continues to dominate, while its population grows. Eventually more than 90% of world Jewry lives in Poland. That culture originally in Babylonia, then in N. Africa, and S. Spain, then watered down in N. Spain, was largely eradicated and replaced with a culture hostile to it.
Nevertheless, from the 16th-19th century, (following the Renaissance), Italy maintained a positive attitude to philosophy and the like, while late 18th century Germany saw a similar shift in attitude. 19th century Italians include Shemuel David Luzzatto, 19th century Germans included R. S. R. Hirsch, R. E. Hildesheimer, and at the end of the century R. D. Z. Hoffman. These trends continued in the 20th century with what became Modern Orthodoxy in America, and are present to a much higher degree in the Religious Zionist world in Israel.
Rather than being based on a chance halakhic dispute over the legitimacy of studying philosophy, as outlined these differing views were the product of radically different worldviews. It seems safe to note that the Ashkenazi anti-intellectualism was more similar to that of their Catholic neighbors, while the Babylonian, North African, Spanish attitude was more similar to that of their neighbors; the Muslim Caliphate, which was for several centuries the world leader of culture and intellectual development.
However, it certainly affected their interpretation of various relevant Talmudic passages. Ashkenazim tended to read them broadly (overly so, as demonstrated by R. Shaul Lieberman z"l in Hellenism in Jewish Palestine). Sephardim, on the other hand emphasized that these statements generally focus on the limiting the teaching of philosophy to children, rather than presenting a wholesale ban. See this article for a discussion of these respective views.
I understand the question to be (1) why the Rishonim were so interested in studying philosophy and (2) why don't we do the same today.
The reason for (1) is because there is much wisdom in Greek Philosophy. The Rishonim such as the Rambam were not so interested in Greek philosophy but rather in sifting the truth from the falsehood in it. The benefit in studying philosophy in general is that, properly done, it allows one to study the Unity of God as explained in the Chovos Halevavos Shaar Yichud.
"The philosopher spoke truth when he said: "no one can serve the Cause of causes and Beginning of beginnings except the prophet of the generation with his senses or the primary (perfect - TL) philosopher with the wisdom he acquired, but others serve other than Him, since they cannot conceive what exists (without beginning - TL), but rather can only conceive that which is composite (i.e. created things - TL)."
(and since there is no prophecy since the destruction of the temple, then according to this the only way to know Gd is through rational inquiry, i.e. philosophy)
Dr. Avraham Apatow, a former professor of Greek Philosophy explains in his introductory thoughts there:
I will share a short story that perhaps illuminates this point. When I came to Jerusalem, I had the opportunity to study with one of the leading teachers of Kabbalah in the english speaking, orthodox world, Rabbi Moshe Schatz. I was in a shiur (class) of his where he presented an overview of the principles for understanding the Eitz Chaim. To my great surprise nearly everything he said in terms of understanding the structure of the Ari's system were principles of wisdom I had learned in philosophy. I asked him where he learned those principles. He told me in the Eitz Chaim itself and he explained to me how. Rabbi Schatz had to learn principles of philosophy in order to understand Kabbalah and he discovered that those philosophical principles were contained within the holy boundaries of Kabbalah.... The mark of a classical education is one that leads a person to experience the awesome nature of the intelligence that pervades every aspect of thought and the natural world in a magnificent harmony. Modern education does not seek to reveal this, because modern education is secular. Classical education was founded upon the perspective that the world is the work of the genius of the Creator, the Almighty G-d. Included in this magnificent creation is the most remarkable work and power, the human mind, the very "eye" that beholds G-d's beautiful handiwork. After studying the creation in all its beauty, the ultimate study is to turn one's attention to beholding the nature of the Creator Himself and His oneness. However, this study is the top rung of a ladder that few have climbed today
(2) the reason it is not studied today is because many Rabbis discouraged it (ex. Vilna Gaon) since it is so easy to err. The Chovos Halevavos quoted about also says "the primary philosopher" i.e. among the most brilliant philosophers of the generation since these matters are extremely difficult to grasp.
There's a famous story attributed to numerous rabbonim (I saw it years ago in the name of Rav Yisroel Salanter, subsequently I've heard it attributed to chassidish rabbonim as well.)
According to the story, a maskil once asked Rav Yisroel how come in the olden days all of the gedolim were also great philosophers like the rambam?
Rav Yisroel answered, "I was thinking about this myself recently. How come in the olden days all of the great philosophers were also gedolim, like the rambam?"
It's a cute story, but I think the point is very true. Very few people can be outstanding in all fields. Partially because there is so much information out there, as mentioned in other answers to this post; and partially because of yeridas hadoros. People are much weaker today and have a harder time focusing, concentrating, remembering etc. (When you hear stories even from pre-war Europe, about yeshiva bachurim routinely learning 10+ hours at a stretch, it helps appreciate how they learned torah back in the day!)
Once we'll say that it's hard to master everything, the first priority becomes learning Torah. Then comes the issue of practical secular knowledge needed for functioning in today's world, including earning a job. (Obviously different community's balance these two differently, and the proper balance probably depends on the individual. but this is for a different post.)
After we've established the need for learning Torah, and the need for practical secular subjects, then we can discuss the question of greek philosophy. Even here- many of the issues are dealt with in Jewish hashkafic sources. I can be familiar with Aristotle's view of the world from reading the Rambam's critique of it.
And again- my first obligation is to have clarity in the fundamentals of Judaism. That means first and foremost, being clear in the Rambam's conclusion, not so much the logical argument he used to arrive there. (Just like in halacha- the first step is knowing what to do. Learning the halachic steps takes training and comes later, hopefully.) Especially when engaging in philosophy, where there's so much room for apikorsus and heresy, it's important to have clarity.
And- there's no schar for ameilus in greek philosophy. Unliked Torah, we don't get rewarded for the effort put in to understand the concepts.
When you put this all together, we get:
1) A much smaller capacity for understanding wisdom (both because there's a lot more out there, and our abilities are less)
2) Greek Philosophy is a much lesser priority than either limud hatorah, or practical secular subjects
3) A lot of the concepts are anyway familiar through the Jewish sources who discuss them
4) The first priority in studying philosophical subjects is making sure we have halachically sound hashkafos, which makes learning the rishonim more important than the greeks who first discussed the idea
5) We don't get reward for toiling in understanding the greeks; at best it could help our comprehension about Hashem and the world, but that requires a lot of time to understand it properly to avoid heresy/apikorsus.
(This could be the underlying point Rav Yisroel was making as well. Not just that it is hard to know everything, but philosophy itself takes people away from the torah because of the heretical aspect. I don't know about this last point.)
In this book, the author mentions that the pious in earlier generations (i.e. Rambam, Chovos Halevaos, Sefere Haikarim) maintened a different level in holiness as well as intellect. Through this elevated level, the were able to grasp the Truth in such a way. However, in the current times, where the intellect is very weak comparatively, many people fail in reaching the Truth in this way. Thus, he proposes, to follow the general Kabbalah in Jewish tradition (Chazalic literature), and simple belief.