It's because when the steel mills collapsed in the 80s, they just won 4 Super Bowls, and everyone left town in a mass exodus. Population dropped by like 500,000. Those aren't bandwagon fans. They're Pittsburghers who got transplanted by a bad economy. That's why they love the Steelers so much, it keeps them connected to the city. Steelers bars aren't for people born in another city who love the Steelers. It's for Pittsburghers who live in another city.
In fairness, you don't see 49ers bars because nobody ever wants to leave Cali. And you don't see Browns bars because most of their fans have killed themselves.