For the simple reason that US believed (rightly or wrongly) that there was more money to be made in concentrating flights out of PHL, instead of PIT. Of course, the O&D numbers seem to suggest that PIT shouldn't have gotten quite the treatment it has, but they seem to look at it as they have a better chance to make money in PHL than PIT (even with WN moving in on both cities). Time will tell if they were right or wrong. PHL is proving to be a continuing issue for US (as we all know).