I think transport in the US might be a factor, though. While Frankfurt-Hahn and Beauvais are particularly extreme examples (they're further from the notional place than most budget airports) you can still get to the the relevant city relatively cheaply (if not particularly _quickly_, in the case of those two; about two hours). In many parts of the US, I suspect you'd really have no option but to hire a car, which instantly wipes out any price advantage of the budget airline.