I've never liked the treatment of Psionics in D&D.
To me, it was just too complicated and too inaccessible. In OD&D, the psionics rules appeared in Supplement III: Eldritch Wizardry, published in 1976. Those rules were badly edited, with psionics rules appearing helter-skelter between descriptions of monks and druids, demons and optional combat rules. They were too complicated, with all sorts of saving throws, table lookups, psionic power recovery algorithms, and new and vague spell-like powers, attacks and defences. Psionics was also too inaccessible, with there being only a rare chance of aquiring psionics (since in OD&D you need a 15 in any one of intelligence, wisdom or charisma, and the chances of rolling a 15 or more is slim to begin with, using the traditional 3d6 method).
Several years pass, and Psionics ends up being relegated to an Appendix in the original AD&D Players Handbook, and receives scant treatment in the AD&D Dungeons Masters Guide. Little surprise then, if it ends up receiving little play at the gaming table.
That was a shame, and a missed opportunity. If Psionicist had been developed as a class, rather than a add-on to the existing classes, it might have gotten more play, and would have helped promote the earlier development of other specialist casters.
But there is a challenge in developing Psionicist as a class. What prime attribute to use? OD&D and AD&D give us at least three potential attributes: Intelligence, Wisdom, or Charisma. In OD&D, a 15 (or 16, in the case of AD&D) in any of those three attributes allows for the possibility of psionic aptitude.
You could allow a high score of 16 in any of those stats to permit the Player to select Psionicist as the character's class. I am philosophically opposed to this approach, because selection of any of the "standard" classes does not require a high prime attribute. I have posted earlier regarding my opposition to the specialist class, Paladin, that requires high attributes to select. I do not like the specialist class restrictions, and think all classes should be accessible to any player with at least a 9 in that classes' prime attribute: the fact that they have low stats in the prime attribute simply means they are not well-suited to that class.
In addition, Intelligence and Wisdom are not appealing to me, as a prime attribute for Psionicists in an old-school D&D system. Not because it doesn't make a certain amount of sense to attach psionic aptitude to those attributes. Rather, it is simply because they are already the prime stats for Magic Users and Clerics.
The only other stat left, of the initial three attributes, is Charisma.