They might well be able to fill it up, but at what fare level. Plus flights from PHL have a much bigger catchment area for connecting traffic than PHX - not many people would fly west to PHX just to go east to Europe/Middle East.
It wasn't mentioned at the crew news session, but another problem is distance. PHX to western Europe (even LHR) is too far for an A333 and 767 without weight restrictions. Throw in the summer temps and the performance penalty that results makes it worse. That leaves only the A332. Apparently they think they can get more revenue from the limited number of A332's flying them from the eastern hubs to Europe/Middle East. while still offering one connection flights from PHX.
Jim