This is a fascinating and excellent question, actually! We know so little about the Ultra Beasts that there’s no one theory that’s more accepted in the scientific community than others, but I have my own. Namely, I think it has less to do with the affects our world has on them and more to do with their specific physiologies. It’s possible that they’re all related in some way—perhaps they’re even actually iterations of the same species, just from alternate dimension. Think of it like you and a you from an alternate timeline.
On that note, I find it quite interesting that they’re all the dominant species of their respective worlds, except for poipole (whose world we came to) and guzzlord (who appeared in an alternate version of our world). If the stories I’ve heard about them are true, anyway.