the region map is quite large but it's just large enough for 3 gens (3 and a half, considering all the pokemon from gen 4 we'll insert). For a single region, that's already a lot more species than any one of the regions of the official games ever had.
and if I were to make a more detailed response: we did think of adding more but it's like, is more pokemon in a single region = better game? No... it's not fun when you go into a route and you're able to find 12 different species of Pokemon in the grass, 5 more while surfing and 5 more while fishing, etc. The areas all become overloaded with species, and it just doesn't feel like a Pokemon game at that point because none of the official games try to cram 7-8 gens of Pokemon into a single region.
we thought 3 gens + a few from 4 were more than enough