>>29641400Crystal and Emerald were essentially glorified romhacks, it would make no sense to release them on a different console.
BW started development long before the 3DS was announced, so there was no way they were going to shift development from the DS to the 3DS.
BW2 had a few circumstances that lead to it being released on the DS. The first was Game Freak came up with a mechanic for BW that they couldn't fit into the game (presumably they were talking about Kyurem fusion), so this was one of the main reasons why they decided to make a third game/sequels. Since BW2 would be set in the same location as BW, they insisted that it was developed for the same console (and it would also mean they don't have to bother porting their engine).
So the only way that we'll get more gen 7 games on the 3DS is if they either make a third version or a sequel. Seeing as this is the 20th anniversary release, they'll probably aim to make this one as complete as possible (i.e. no additional mechanics or storylines that they can expand on in another game) so I don't see them releasing more gen 7 games.
It's also worth noting that all previous handhelds had backwards compatibility with each other, so owners of the new consoles could still play the new Pokemon games. The Switch doesn't look like it will be backwards-compatible with the 3DS (seeing as it only has one screen).
