>>44198887Kanto, Johto, Hoenn, and Sinnoh were all in the same immediate vicinity, so it would make sense in retrospect that we saw no regional variants there.
Unova was entirely populated by its own species, so same thing.
Kalos... there's a bit less of an argument about.
In Alola, it had nothing to do with "dimensional portals and magic ground." It was literally part of the theme of invasive species, which was reflected in regional variants, Ultra Beasts, and Pokedex entries about various Pokemon (including ones that we hadn't seen before Gen 7).
There's not a particularly strong argument against other regional variants being able to exist.
