Why are Philadelphia sports fans so rabid?

Answer 1

I think you will find rabid fans in every major metropolis that hosts a professional or college sports team.

Fans in Philadelphia might seem more rabid than say, Seattle for perhaps several reasons.
One reason might be simply the length of time a team has been in a city. Time provides the opportunity for successive generations to grow up being a fan.
Decades of tradition hold stories, legends, hall of famers, and championships. The team reaches icon status. An entity unto itself.
You might still find Baltimore fans rooting for the Colts, but it is unlikely that you will find too many Chicago denizens rooting for the Cardinals because of their former tenure in the Windy City.
Reasons for being a fan of a team are varied, and probably best discussed in a psychological context, but people enjoy competition, and people enjoy even more feeling like they are winners. Fans contribute very little to the actual outcomes of most games, but they still invest themselves heavily in their team's success.

Again, Philadelphia is not unique in its rabididity. (I know, not a real word). One look at the fans in any home Raiders game should attest to that. Or the Hogs fans in DC. Or the Cheeseheads. Or the Gators in Florida. Or the Huskers fans.
Or, if you really want to see rabid fans, attend a 'football' match in the UK. They dont call them soccer hooligans for nothing.