In some cases, whole states and even entire regions of the country uniformly support a single team. For instance the Vikings are easily the only game in town in Minnesota, while New England (minus New York) appears to be comprised of entirely Patriots fans except for a small portion of Connecticut.
There are some states which are divided into regions by teams. Florida has three teams--the Tampa Bay Bucs, Miami Dolphins, and the Jacksonville Jaguars--and Facebook users there seems fractured in their support, with some counties even defecting to teams from the North. Ohio is another interesting story, with the Cleveland Browns in the North, Cincinatti Bengals in the South, and Pittsburgh Steelers fans occupying the middle of the state.
Some teams, like the Steelers, Cowboys, and Packers, seem to transcend geography, with pockets of fans all over the country. On the other end of the spectrum, the Jets have to share New York with the Giants and are only the most popular team for a single stronghold county in Long Island.
Emphasis added. Fans of the Cowboys and Packers transcend geography because they are successful franchises. The same explanation applies to Steeler fans. But that's only part of the story. Pittsburgh excels at talent production. As a result, the brain drain during the 1980s was epic.
There might be a great outmigration for each decade of US history. As noted above, the exodus from Southwestern PA defined the 80s. In the 90s, it was Californication. The noughts birthed the Katrina Diaspora. Steeler fans are the Okies of today. Neither group was welcome in their new place of residence.
The Daily Mail picked up on a geographic oddity:
An enclave of Pittsburgh fans in Oregon? The surprising map that shows how the country is divided over the NFL
Someone responded on Twitter with an answer for the question, "Not surprising at all. Combo of all-time success plus 70s-80s rust belt diaspora."
That Tweet is what brought the map to my attention. It is a strange enclave of Steeler fans. I doubt the 80s exodus has anything to do with it. If a different NFL team drafted Troy Polamalu, then the Oregon enclave would be a different color.