^PHL is much more limited in expansion than CLT is, PHL is already landlocked and one of the most congested hubs in the country, I think it ranked among the top 10 airports for most delays last year. The terminal and runway layout there also doesn't allow for much expansion. CLT on the other hand already has four runways and can accomodate a terminal expansion. Plenty of ramp space there too to go around for additional expansion.