That's partially the same problem as the Aberrant Mind sorcerer. I want psionics to matter in the world. I want there to be a multitude of different types of psions, specializing in various types of powers. I want psionics to have at least as much breadth and variety as magic does. And I want them to be distinct from magic. Not necessarily 2e-level "psionics and magic do not affect one another unless specifically stated" – I'm completely OK with e.g. dispel magic working on a psionic power. But it should work in a distinct fashion.
Playing a reskinned diviner does not do that. It would suffice if I was running a one-shot with pregens and I wanted one of the characters to be a psion. But it does not work for a world where there are multiple different schools for teaching people to develop psychic powers, and with multiple psionic factions around.
"But that's not how the Forgotten Realms is!", some would say. To which I say, "I don't care, because I'm not really interested in playing in the Forgotten Realms."