2 out of 5 have had a Psion class. Two. out of Five. One of those two was the least selling edition of D&D. I think we've had as many official Jester classes as we've had Psion classes. And this is what is being described as not just a tradition, but a "strong tradition"?
Very disingenuous.
Dragon Magazine 78 had an unofficial class that became the template for the 2e Psionicist class. There were two versions of the 2e class actually: the first being in the Complete Psionics Handbook (and the one most people think of) and a revised version in Player's Option: Skills & Powers that used a different "AC vs Thac0" like system instead of ability checks. Third edition likewise had two vastly different versions of the psion(icist) class; the 3e version that had random d20-based save DCs (and each discipline tied to an ability score) and the later 3.5 version that is basically a spell-point 5e wizard. 4e likewise had a psion class, build around ADEU like all 4e classes but omitting Encounter powers for PP to augment at-wills. So, depending on how you take the canonical status of Dragon, a character class that has focused on just using psionics has existed for four editions.
The far bigger issue, IMHO, is that in four editions the damn class has looked extremely different from one another and in two editions got major overhauls to fix how utterly broken it was. There is little that unifies the various psionic classes beyond "points-based resources" and "vaguely psychic abilities like TK or mind-reading" Everything else has been in flux; weapons, armor, HD, specific powers, attack/defense modes, etc. Wizards, for the most part, have looked similar each edition, or at least have kept similar trappings. Psionics hasn't stayed the same within the same edition for most of its life.
So, I kind of don't blame WotC for giving up and opting for subclasses, TBH. Psionics, outside of a few key powers like telepathy, teleportation, telekinesis, and clairsentience, has never kept a solid identity. And it really looks like WotC is not keen on a whole new subsystem for psionics anymore. That more or less puts them back to the point of having psionics be an augment to classes power rather than its own class. Simply put, they don't see much value in re-inventing the wheel yet again, especially with how the Mystic was received.
As I said, they are probably going to Paizo it; here are a few psionic-feeling abilities characters, and if you want a unique system, look to a third party for a solution.