Why are kabbalah teachings called secrets? Why were these teachings hidden for so long?

Because whatever secrets there are in the teachings of Kabbalah, they are not to be found in books. If it's in a book, it isn't secret. 

 

Teaching Kabbalah is not about sharing information. It's about triggering memories locked in the mind of the student. And it is about the student asking the question that unlocks connections in the mind of the rebbe.