Philadelphia suffers from being too close geographically to Manhattan. You already have Seattle, LA, Denver, Manhattan, DeeCee... You sometimes get stuff on Atlanta or Cara'sir, since those are national capitols. But that's a lot of coverage for sprawls in the US, and then you have NeoTokyo, Hong Kong, Lagos, all of Europe, whatever parts of Africa you feel like running scared from, Australia, Aztlan, Amazonia, and so on. And don't forget Space, what with ZO up there, and the Monads on Mars. Oh, and there's the stuff in Antarctica...
So yeah, lots of stuff to cover, and not many books to do it in. So Philly doesn't get covered much. Hell, Detroit doesn't get covered much, despite (or because of) it being a AAA's world HQ.