The more interesting general question is "why do different types of settlement vote as they do?" Unfortunately, like many partisan questions it inevitably spirals down to hackery. According to Republicans, exurbs vote Republican because they're full of affluent people who want good schools and safe communities for their kids. According to Democrats, exurbs vote Republican because they're full of insular, bland whites who want to keep their kids away from minorities. There's probably truth to both statements, except both sides will always play up the story that satiates their preconceived world view.
Well statistically the two are probably one in the same. Good schools and safe communities
are generally in what we might call bland areas, and minorities typically populate in inner cities, where schools aren't as good and commnities have high crime. It's the 1950s white flight to the suburbs all over again.
