But why? Besides from actual rallies, appearances, etc, which I doubt have a very significant effect on people's votes (people only go to the rallies of their preferred candidates), there is no reason to target people in big cities. Given that most campaigning happens online or through the media at this point, there is no reason to restrict yourself to geographical areas.
But they do ignore the rest. How much campaigning was done in rural Texas or California? Or Wyoming or Montana for that matter?
It's silly to pretend that Michigan and Florida somehow represent the country folk of America.