Why was the Book of the Zohar hidden to the first generations?
The Zohar was hidden during nine centuries between the 4th and the 13th. This happened in spite of the fact that those generations were much more spiritually mature and by far deserved to study this book. This spiritual maturity stemmed from the purety of their desires. That is why they did not need the book of Zohar to further their correction.
Why there were no valid commentaries of the Zohar before the Ari?
Simply because during 13 centuries (from the 4th to the 16th), between the Rashbi's and the Ari's time no Kabbalist ever reached the required level to clarify the Zohar and Kabbalah as a whole. The Ari, Rabbi Isaac Luria (1534-1572) explained the fundamentals of Kabbalah but never wrote any commentaries of the book of the Zohar. Only from the 16th century did printed versions of the Zohar start being circulated.
Why haven't there been any commentaries of the works of Ari and the Zohar between the Ari's time and today?
There haven't been any commentaries similar to the Sulam's for centuries and on Ari's works during 3 centuries. Only when came the time for the diffusion of Kabbalah did a particular soul incarnate, Rabbi Yehuda Ashlag's . He transmitted the science of Kabbalah in a way which could be understood by our generation.
This does not mean that there were no great Kabbalists before our generation, on the contrary, but what they had learned could only be used in their private work. They were not allowed to teach Kabbalah.There were Kabbalists before the Rashbi whose knowledge was just as impressive but they did not have the right to reveal the secrets of Kabbalah.
The fact that today it is possible to study Kabbalah and enlarge the circle of its students (in accordance with principles of the Ari) bears witness to the Creator's wish to grant our generation the strength leading to the achievement of correction.