The core issue with psionics is that there are several possible incompatible implementations, and the people who are least satisfied with the current situation are the ones that want the psionics that'd cause WotC the most trouble to implement.
5th edition has already evolved a psionics system largely equivalent to that presented in OD&D and AD&D 1st edition, in terms of role if not remotely in mechanics. The feats and subclass features as of Tasha's are the fairly obvious match for the occasional, mostly-not-character-defining psionic powers in those editions.
The second choice, psionics equivalent to the 3.5 XPH and Dreamscarred Press psionics for PF1 are, in fact, pretty much just spells. A quick way to do it is rename the 5e sorcerer class "psion", use the DMG spell point option for this class, create new subclasses for different flavors of psion, modify the spell list a bit, and replace the spells' verbal components with audio manifestations and somatic components with visual manifestations. People who want this, and would be satisfied with this, but aren't currently happy, do exist. But they aren't the people who make the most noise on the boards about wanting psionics.
The third, a whole new system that doesn't work like spell magic, and is broad and complete enough to support a psionicist "full caster" class, is what was attempted in AD&D 2nd edition (twice, actually). The first problem from a WotC perspective is that making a system as broad and flexible as spell magic is as difficult to balance as the spell magic system, and that effort was huge and took a long time. The second problem is that by adding such a broad and flexible system to the game, you are massively increasing the total complexity of D&D, which will result in massive rejection by people who were near their complexity limits with D&D as-is. The third is that there's no natural niche for psionic effects in the game; to make this new system not completely redundant you'd have to claw back powers already assigned to various forms of magic to create one, which would also cause massive rejection by existing players of magic-users. The fourth is retrofitting such a system to existing settings in a way that feels natural rather than tacked-on -- or else dooming the whole psionics project to being a red-headed stepchild.
In the Level Up Voidrunner's Codex project, the creators have seriously reduced their issue because they have completely tossed the traditional magic system aside for their project. They don't have to worry about corner case interactions with spell magic, because they've tossed out spell magic. They don't have to make sure that spell magic and psionic "full caster" classes are equals, because they've tossed out spell magic. They don't have to worry about the complexity burden of two supernatural powers systems in a game, because they've tossed out spell magic. They don't have to worry in about niche overlaps and protection between psionics and spellcasters, because they've tossed out spell magic. And they don't have to worry about existing customers' reactions, because instead of introducing psionics to an existing game and retrofitting to existing material, they're making a new game. Accordingly, they have a vastly easier task than WotC would in adding a complete psionics system to D&D, both as a matter of game design and as a business managing a fanbase.