The Mesillas Yesharim is certainly one of the most influential and popular seforim ever written. It is considered a basic text in most yeshivos and is widely studied by Jews throughout the world (both in the original language and in translation).
Before we can address the reasons for the immense popularity of this work, we first need to address one of the specific concerns mentioned in question. The question mentions that accusations were made against the Ramchal (R' Moshe Chaim Luzzatto, author of Mesillas Yesharim) in his lifetime that for improper involvement in mystical practices and even Sabbatianism (adherence to one of the heretical cults based on the false messiah, Sabbatai Zvi). The question asks how it can be that a work from such a controversial figure could have become so widely accepted within the traditional/Orthodox community.
The reality is, however, that the Jewish community has accepted that these accusations were incorrect. The fact that major figures such as the Vilna Gaon strongly endorsed the Ramchal and his works served to completely clear any suspicion from his name. The conventional opinion within the Orthodox community is that, as is often the case with complex figures (and the Ramchal was certainly a complex figure), especially those deeply involved with kabbalah and mysticism, the Ramchal was simply misunderstood. (The Ramchal was not unique in this regard; R' Yonason Eibshutz is another example of a major figure accused of Sabbatianism, whose works are fully accepted today, and for largely the same reason.)
Thus, the accusations against the Ramchal have not been given any credence within the religious Jewish world for well over two centuries, and are simply considered an unfortunate aspect of history.
Of course, the fact that the Ramchal is not viewed as a controversial figure does not, in of itself, explain why the Mesillas Yesharim became so popular. In fact, the Ramchal himself wrote many works, and while some are fairly popular, none of them comes close to the extraordinary popularity that the Mesillas Yesharim has enjoyed for more than two centuries.
I don't believe there are any definite answers to this question. Clearly, the fact that the Mesillas Yesharim was enthusiastically endorsed by numerous major rabbinic figures over the years has played a large role in its general popularity. However, asides from the fact that this does not explain why the book received such enthusiastic endorsements, it also fails to really explain the work's general popularity as well. There have been many works over the years that have been enthusiastically endorsed by major figures, that failed to really gain general popularity. The main reason people are aware of the endorsements given to the Mesillas Yesharim is that people are reading and talking about the book.
So what was it about the Mesillas Yesharim that set it apart from all the other classic mussar works, none of which enjoys a popularity that even approaches that of the Mesillas Yesharim? In my opinion, there is one major factor, above and beyond all others, that made the Mesillas Yesharim a truly unique work in its time, and which continues to contribute to its immense popularity.
This factor is that, unlike virtually all previous mussar works, the Mesillas Yesharim refrains entirely from both harsh, condemnatory language directed at the reader, and also refrains from lengthy technical discussions. On the contrary, the work continually stresses that every positive step, no matter how small, is actually a major achievement, and that even one who attains to only the lowest of the levels described in the book has done something extraordinary.
With many earlier works, one can walk away with a sense of despair or even fear. (An very religious elderly woman once told me that she could never read Shaarei Teshuva because it was too frightening.) Other works, such as the Chovos Halevovos, are written more in the style of philosophical works than as guides to self-improvement. In both cases, what the reader does not get is the sense of a supportive and understanding mentor that clearly comes through with the Mesillas Yesharim.
In writing the Mesillas Yesharim in this manner, the Ramchal demonstrated a deep sensitivity to the changes that were taking place in society (both Jewish society and society at large). While earlier generations apparently found the older style mussar seforim effective, in the modern world a very different approach was necessary. The Mesillas Yesharim was thus, in many ways, the first modern Hebrew work. (Indeed, the early maskilim were great admirers of the Ramchal, and the Mesillas Yesharim, for this very reason.)
Given this, it is not surprising that the Mesillas Yesharim was a huge "hit" and became the most popular mussar work of all time.
There is one more issue that is raised in the question that needs to be addressed. Anyone who has studied the Mesillas Yesharim will quickly recognize that, of the nine levels he describes, few people ever attain much beyond the first two. That being so, what is the point of studying the later sections?
I recently discussed this topic with my rebbi, who is himself a long-time student of the Mesillas Yesharim. In the discussion, we came to three basic reasons why the later sections of Mesillas Yesharim are relevant to every Jew.
- "You can't begin a journey if you don't know where you are going." Meaning, even when you are still on a lower levels, the knowledge of what you are working to eventually achieve on the higher levels still has a major impact.
- Spiritual growth is often uneven. There are always some areas in which we are stronger than in other areas. Thus, it is possible that while a person may have only achieved the first or second level in certain regards, he is nevertheless on level 5 or 6 in other areas. A person should not restrict his spiritual growth in one area, while he waits for his weakest areas to "catch up".
- It is important for us to understand what true spirituality is, so that we will be able to recognize it (or its absence) in the people around us. Even if we have not achieved the highest levels described in Mesillas Yesharim, if we study them we will at least be able to recognize such greatness when we encounter it in another person, and we will also be able to recognize its absence in those who put on false pretenses of holiness.
