Farm system for mayors?

Don't know why I thought of this, but is it odd that mayors don't seem to move around very much? Right now in Boston we're choosing between essentially three candidates, all of whom are local -- the current mayor, Menino, and two councilors, Yoon and Flaherty. My question is, why isn't there much of a farm system for mayors? Something like, I've been mayor of Portland, Maine for the past 8 years, and now I'm ready for something bigger, so now I'll move to Boston. And if you do a good job in Boston, you run in Houston, and then New York.

Certainly one pragmatic reason is residency requirements, which I'm sure exist. But it does strike me as odd that we don't seem to include people who have actually run cities before, just not our city. Put another way, 'd imagine that being a good mayor of town B is a better predictor of success at mayoring town A than being some sort of lower official in town A.