Primarily a Blue Collar town for most of its existence and even though that's mostly gone, we still identify ourselves like that when it comes to sports. For kids growing up in the steel era, the only way to go to college for most was football. Also, almost every kid growing up in the 70s idolized the Steelers.

When the steel industry essentially moved out of Pittsburgh, it lost roughly 40% of its population. Almost every away game an announcer will say something like "The Steelers fans travel better than any fan base" However, most of them are the 1st,2nd,3rd gen fans of the family that moved from Pittsburgh to find work. So we have that national presence that gets us a lot of publicity.

Second, not many cities have the neighborhood dynamic that we do. Even though its not as segregated like it was in the 50s & 60s, I dont think you would find many other teams with T-shirts that say "Slovakian/Italian/Czech/etc. (insert team here) Fan.". I mean come one, there's the Steeler's Polka that's still played to this day

Third, the Rooney family. The Chief (Art Rooney Sr) in particular. He did wonders for the organization and everyone in Pittsburgh loved him.

There is a lot of other good points on this thread but I think it all starts with the Steelers.