Answer: First, Kabbalists did not hide anything. They have always been for the revelation of this wisdom.
But 2,000 years ago, after the destruction of the Temple, people began to turn their backs to the wisdom of Kabbalah to such a degree that Kabbalists were forced to hide The Book of Zohar and other sources so that they would be reveled in our generation.
Baal HaSulam began to adapt the wisdom of Kabbalah to our times and only today is it being revealed.
From the Kabbalah Lesson in Russian 3/19/17