Business Insider: Facebook published a map showing NFL fandom by county, and some of the results may surprise you. For one, the pocket of Pittsburgh Steeler fans in Douglas County.
Geography is a hugely important factor in determining team allegiance. But that's a little misleading. There's nothing magical about being born in a specific place that makes you a fan of one team or another, it's more about culture than place.
Fandom comes out of cultural pressure — you become a fan of Team X because your friends and family are fans of Team X and Team X is a prominent fixture in the lives of the community in which you live.