Schools -- We're trying to provide the highest possible quality education for our daughter, which means living in the best public school district we can find or within range of the best private school we can afford. And while I'm sure there are exceptions, this means living in a city.
Jobs -- Even though in theory I could work remotely, my wife (a lawyer) couldn't. Wherever we live has to be able to provide employment opportunities for two disparate professions.
Arts/Culture/Etc... -- I take my daughter to the Thinkery (a neat kid's museum here in Austin) monthly. She takes ballet and swim lessons, perhaps music lessons soon. It's important for our family to be in proximity to a vibrant art/music scene.
None of this is meant to denigrate small towns or rural areas, and perhaps there are some areas that provide the above (I'd be curious to hear about them if there were, actually...) I merely mention this to indicate how difficult the project is.