The study, highlighted over at Discovering Urbanism (go there to see a good chart), found that a trend of reinvestment in urban centers throughout the country is continuing, and city residents are becoming richer in comparison to their suburban counterparts.
That is, all except for two: Detroit, which has had to deal with the collapse of the Big Three automakers (and is just generally Detroit) and ... Philly. Even Washington, which saw its share of disinvestment, has reached income parity with its suburbs.
Discovering Urbanism doesn't speculate about why Philly might be lagging behind here. Neither does Planet Money.