>>54859214The ingame lore explanation was that oldgen mons started appearing because of picrel and the Pokétransfer related experiments. Of course, unlike ORAS where a new batch of species starts spawning all over the region after the Primal fight, in BW the spawns don't change at all throughout the game. But since all accesses are blocked you have no "legal" way to check there, so you can pretend those areas were just full of unovamons.
By B2W2 the regional dex is already expanded to include all of those.
