Answer: First, these books were written by people who had attained the upper world. Second, all of them are called Kedoshim (sacred), and Kadosh denotes distinction, uniqueness.
The Book of Zohar is unique because it was written by a group of ten Kabbalists: Rabbi Shimon and nine of his students who attained the highest level of attainment of the upper world. From this level, they fully described their ascent and all the details of the spiritual ladder, the ladder for attaining the Creator.
The Book of Zohar is appropriate for everyone because it was written for our generation from the interconnection of ten people. For a long time it was hidden and became accessible to people a few centuries ago, but its inner meaning was discovered only in the beginning of the 20th century by the great Kabbalist, Baal HaSulam.
Question: Was Baal HaSulam the only one who interpreted The Book of Zohar?
Answer: Other interpretations exist, but they do not explain the entire spiritual system. Baal HaSulam wrote this explanation when he was on the same level as the authors of The Book of Zohar. The rest of the Kabbalists did not attain this level, and therefore, they could describe something, but no more than that.
From the Kabbalah Lesson in Russian 2/5/17