The changing of one's mind, Mind of Tempest.
In this instance, note how the Unearthed Arcana psionic subclasses that might have encroached upon a psionicist, such as the Wizard subclass, were changed. I think room was left for a future development.
When Mike Mearls was discussing psionics years ago (I think during his "Super Happy Fun Time" videos), he broached the notion of having a psionic subclass for each class in addition to having a psionicist class (which I thought was brilliant). Obviously, it is possible that the notion of the psionicist was completely scrapped forever and ever and only the subclass idea has been implemented, but nothing from the lips of Jeremy Crawford at the time of the release of Tashsa's or when the psionics subclasses were released on Unearthed Arcana suggests that. Rather, his comments suggest merely that the timing is not right.
The only thing that we have heard is that the Unearthed Arcana version of the Mystic did not receive enough positive feedback. I have observed that Wizards likes to keep its ideas and return to them. I credit them for staying with an idea until it works.
But, that's what it is like under my hat. Some people do not share that optimism.
My sense of the people working on D&D is that they love the game and love the history of the game. I have little doubt that, given time and the correct project, many of them would jump at the opportunity to develop a psionicist.
