there's probably a section of Kanto that he didn't visit, with more diverse Pokemon.
think about it this way; the anime is based off of the games. in the games, you play as the character who visits certain towns and battles certain gyms. you just happen to start in Pallet, and go to the eight gyms we all know and love. so Ash followed the games' path; Pallet-Viridian-Cerulean etc, while in reality there is much more to Kanto than you think.
does this make sense?
