Actually there are two reasons, one is that it has Religious connotations with Hinduism and we don't want that do we! And the other, is that it is a very high state of development and few meditators (of any discipline) ever experience it. Why even mention it, TM is Yoga-lite for modernity anyway, and that's good.
That being said, I think it was deplorable that he left without giving a clear teaching and understanding of these fundamental principles, and to think TM could be considered a complete Vedic system, unreal! OK granted, they may call them something different like nadis or whatnot, but come on we all know what they are, chickens! :-) PS. It forces TM'ers to look else where for answers, like Sri Sri anybody? http://www.youtube.com/watch?v=uofjzGoR8JA
