>>39479133I'm a developer and at one point I tried a hand on 3D modeling.
And rest assured, this has little to do with them not having the manpower to bring all Pokémon back. If they cared, DLCs are a thing. Or they could promise that future games will have all Pokémon available, while they keep working on future proofing the models, given they had to do them from scratch (which is an utter lie of course). There are MANY solutions to this whole problem.
But they picked the one that caused the biggest backlash: coming forward, all Pokémon will not be available in all games. The reason is unknown, but if this isn't a technical issue (which is not, Switch cartridges have a lot more of space than 3DS ones), then it can only be a commercial one. Something else drove them to cull the dex.
I'd suspect diminishing returns, or just some sort of move to earn more money, who knows.