Why are they 'hidden'?
My guess is that unless a rabbi is a public teacher, there is no reason to publicize his learning.
'Kabbalah' is from a root meaning 'to recieve' and all of traditional Torah is a 'recieving' from teachers. The better expression might be 'sohd', which means 'secret' or 'hidden'. It is 'hidden' in the sense that it is very difficult to understand - like complex mathematics - and easy to misunderstand, because it uses words (that sometimes have ordinary meanings) instead of specialized symbols.
Rav Kaduri (Google him) is an example of a true 'kabbalist' who is known. Most public 'kabbalists' are just huxsters. Many more students and masters just learn quietly.