Then it's not people "keeping a psion from being developed". It's that nobody has come up with a clear picture of what a psion should be in 5E. Big difference.
Let me rephrase that then. I have seen a handful of people defending WotC's inability to come up with a clear picture for the Psion in 5e and using that as an excuse why the Psion should not be a class. No, I don't think that this is any anti-psion illuminati, but it does feel a lot like white-knighting WotC while also talking down discussion of how the psion could exist in 5e. And similar issues plagued both the chances of a revised Ranger and the creation of the Artificer class. Thankfully many of those same critics of the Artificer ("nobody knows what the artificer should be. LOL") are now mostly silent with the release of the Artificer in Eberron 5e.
So I'll ask my question again. What would a psion look like other than a sorcerer/warlock/wizard with different fluff?
The Wizard has a spellbook that empowers their versatility. They can potentially know all the arcane spells, but they can only prepare a limited number per day. The Sorcerer has a much smaller spell list and spells known list, but they (controversially) compensate this weakness with more cantrips and Metamagic points that allow them to augment their sorcery. The Warlock has Pact Magic, so they cast on an auto-scaling per Short Rest resource, but they have a lot of build diversity (on paper) by their ability to pick a Patron, a Pact Boon, and Invocations, including some at-will/per day abilities and things that expand/supplement their tool set.
So where could the Psion fit? Potentially a little bit of Column A, a little bit of Column B, and probably with some smatterings from the Monk. I am entertaining several ideas in my head, namely the Monk and Warlock as a base or possibly a mix between the two. It depends on what themes of the Psion you want to emphasize and what mechanics you think works best for the fluff.
Spell points? There's a variant rule in the DMG.
Ki or something like a Warlock's pact magic would work. This would put the Psion on a similar mechanic to one of two pre-existing classes. So either the monk stops being alone or the warlock stops being alone. This would amount to more than re-fluffing the Warlock since the Warlock has two decision points: Patron and Pact Boon. We can reduce this to just a single subclass option for the Psion.
What spells or powers would distinguish them?
Psychometabolism, astral constructs, additional mental spells, crystals, more spells that inflicted psychic damage, ectoplasm, etc.
What would the design goal of a psion be that would differentiate it?
That sounds like fluff, since we are talking about how the mechanics would reflect the fluff, aesthetic, or theme of the Psion. The Psion is less about educational book learning for mind over cosmic matter as per the wizard but more about the mind over matter as part of their mental mastery. In this respect, there is definitely overlap with the Monk, but the Monk wants to hone their martial skill and body, oneness with the universe and life. The Psion wants to hone and expand the powers of their mind, mastery of their own mind. Maybe it's innate? Maybe it's learned. Either way, it extends from the mastery of their own mind.
So for Psion I would probably prefer more at-will and per Short Rest mechanics than a traditional full-caster, since they will undoubtedly be working from a smaller spell/power list. A monk's Ki and a Warlock's Pact Magic are both on a per Short Rest mechanic, which is why both could work. Ki are more like Power/Spell Points and Pact Magic is a spell auto-boost mechanic.