It's because of a little thing called the "save data cloud backup" where save data is stored on you guessed it, the cloud
All of the NSO apps support SDCB whilst non of the main pokemon games (possibly excluding legends za since I don't have it) support it.
So by this understanding I probably think that if they do re release the old games, it would be in one collection or as a standalone software like the galaxy games
(I have no clue why non of these things work it's just that's why I think they haven't came yet)
