@Diablerie
Well, there are only so many things you can do in one region. So, if you have a lot of regions, theres a lot of things to do. In Isshu, you can go to the Dream World. Can you do that in Hoenn, Kanto, Sinnoh or Johto? In Isshu, you can catch a Zekrom. Can you do that anywhere else?
TL;DR: More regions, more pokemon, bigger variety of things to do.
BTW, I don't think Awakening In A Room counts as a CYOA because, the title does not say 'Choose Your Own Adventure'.