I assume its because they feed US's international operation. And PHL has significant O&D on its own, and feed to ORD and IAD to UA's own operation has to be there. Example being Asia: PHL has no *A Asia service (right?) and US flyers in C or F can easily fly UA metal to ORD, LAX, SFO or even SEA (one stop, bad example maybe) and will take up UA's F inventory.

I've noticed that domestic F has been on the rise, and QUPs are becoming a little more difficult. For me at least, the difference has been in refundable vs. nonrefundable QUPs. The only routes I purchase F as a student are SAN/CLD-ATL and SAN/CLD-SEA, and have struggled to find refundable QUPs less than 2 weeks out for the days I need to travel, but can still find restricted, like QUAUP7N, which sorta sucks. Of course, finding out someone is sitting in my preferred seat is just as frustrating, but what can you do?