Probably nothing more than incumbency.
That's really too simplistic. Santorum had that advantage, yes, but Klink was underfunded and couldn't mobilize support. He was well known here in the west and a good fit. He used to work as a newscaster on KDKA. But what is that to people in Philadelphia? He had no name recognition and no money to push at the Republicans who poured money into Santorum's campaign. All Santorum had to do was do well enough here and hold the suburbs and he did that. Incumbency is a part of it, not the whole picture.
