Here's how I personally see this (based on the tradition of my Rabbi) - it takes two to tango. This is based on a famous parable about the Moon (Chulin 60b):
רבי שמעון בן פזי: ... אמרה ירח לפני הקב"ה רבש"ע אפשר לשני מלכים שישתמשו בכתר אחד אמר לה לכי ומעטי את עצמך
Rabbi Shimon ben Pazi... explains: When G-d first created the sun and the moon, they were equal. Then, the moon said before G-d: Is it possible for two kings to serve with one crown? G-d, therefore, said to the moon: If so, go and diminish yourself.
This parable is frequently compared to the dynamics of this world, which is based on the interaction between G-d and the Jewish people, G-d being the Greater luminary and Israel being the Lesser luminary. As you might see, that just as all the light that comes from the Moon is just a reflection of the light of the Sun, similarly the wisdom of the Jews is a "reflection" of the divine wisdom.
As we know the Torah is divided into two parts - the Written (the Writings) and the Oral (Mishnah, Talmud and on). While the first was received from G-d (either dictated or by Ruach Hakodesh), the other is the product of the Rabbinical thought (i.g. the "reflection"). (Note in Kabbalah, the former is called ז"א and the later מלכות).
Similarly, the times (of the 6000 years of the creation) are roughly divided into 3 eras of 2000 years (based on Avodah Zarah 9): 2000 years before Torah, 2000 years of "the Written Torah" being the main part and 2000 years of the Oral Torah playing the main part. Here's where the tango example kicks in - after the receiving of the Torah, there is an epoch when G-d leads and an epoch where the Sages lead.
The epoch of the prophecy lasted until the beginning of the second Temple. That was the time of the Divine light, so to speak, where [practically] no Halachic arguments existed - the prophets could reveal the truth to anyone. The written Torah served as the ultimate source of knowledge and everything Halachic being derived from it by accepting the Divine messages. But the problem was that in the light of the Prophets no real Rabbinic thought could be expressed as it becomes unimportant.
However, it is not fun for G-d to lead alone and He "wanted" to reveal the light of the Sages. But as we know "שרגא בטיהרא מאי אהני" (Chulin 60) - the light of a candle cannot be seen in sunlight, therefore to let the Sages' light shine G-d needed to withdraw His own light (aka prophecy) and become "Hester/Hester Panim". That's how the epoch of the Sages started (with the revelation of Purim - קבלו ברצון) and continues till nowadays.
Therefore, as I showed, the end of prophecy did not occur "randomly" or as a result of "ירידת הדורות" but was "planned" from the beginning of the creation, just as the destruction of the two Temples was planned (as Rashi brings on "והוא בכה על צואריו" Ber 45,14) with the sole purpose of enabling the light/the wisdom of the Sages (and Tzaddikim) shine.
I hope it "shades some light" on the necessity of the ending of the era of prophecy.
NB. 1. Please don't downvote anonymously just because you've never heard those ideas.
2. I know, some may argue that Chachamim existed from the times of Moses. True, but they always were subservient/secondary to the leader of the Nation and not as prominent as the Sages in the times of the Second Temple.
