A user on Chowhound posted in 2006 about the origins of Philly pizza. It might be a few months old but it’s well worth reading.
As we get it, the gist of things is that Philadelphia pizza largely escaped the Italian style of thin crust pies normally associated with New York City and New Haven. Instead, Philadelphia pizza owes more to Greek pizza—a method of pizza making that emphasizes a thick, chewy crust, sweet sauce and cheese that oftentimes mixes in cheddar alongside mozzarella.
Greek-style pizza is also the dominant variety of pizza in Boston, another city with a significant Italian-American population. Our personal guess is that this has a lot to do with the popularization of pizza as a “non-ethnic” food after WWII; given that Greek immigration to the States and mass openings of restaurants only happened after the war, this may well be the case.
Chowhound poster mbetoni mentioned several classic (and quality) pizzerias in Philadelphia:
Family Style Pizza in Southwest Philly is the “pizza that probably epitomizes the Philadelphia-style. Celebre’s by the stadium is some of the best pizza in Philly and is even better when pies are ordered well-done. Marra’s, meanwhile, is the “best pizza in Philly, hands-down”.