Why did Ash Ketchum ever leave the Kanto region?
What a way to nitpick a single sentence out of my post where my next sentence starts the explanation.
Now read on from there.
As far as Ash Ketchum leaving to other regions, it's only to continue the Pokemon anime storyline with the same character. He wanted to travel the world, and there were some storylines to the new region (weak, maybe, but that's not the point). For the Pokemon games, to have all regions, travelling it with one single character just for the sake of travelling might not be enough. It might be a little more logical in an official Pokemon release to have all regions, where catching Pokemon actually matters. That way you can catch every single Pokemon (barring certain legendaries) with one single game.