Because I think the rest of the world has a lot more story possibilities, and the whole "All superheroes are in the U.S./NYC" thing is a bit done to death :)
those are both good points, and I do think that the X-men should think of the whole world as their jurisdiction, but isn't that what they have jets and teleporters for?
don't get me wrong, I usually think it's cool when the X-men move somewhere new, I liked a lot of early Excaliber for that, Muir island was cool, SF, Australia, even Utopia was interesting, conceptually. I just don't think that where their based out of should matter to where the stories are set, or where their missions take them.
I mean, the concept of there being a school seems pretty key, but other than that, the X-men travelled around the world and across the universe before they ever changed bases. And the first time they did, they really lost something by separating themselves from the school (which, to my mind, should be their first priority).
All that said, I have been wondering if Storm might end up enlisting the aid of Gateway or someone so that she can base her team apart from the school. Psylocke's not supposed to go to the school now, apparently, and who knows where Rogue will be based if she's on the X-men and the Avengers. She could base the team out of the outback, and just set up agents all over the world, for if and when the story takes them there. To me, that just makes more sense.