For flights to Europe, JFK is pretty much on the way. So routing (connecting) there doesn't take people away from Europe first. ORD, depending on the origin city, can be convenient or out-of-the-way. Both are more hassle and delay prone than CLT, admittedly. To me, the one thing that may weigh in PHL's favor is the amount of O&D at NYC - another airport with a decent amount of O&D could be useful for additional service to European destinations.
ORD serves a pretty large area as well as JFK does plus it also has a rather large base of O&D traffic. Unfortunately, CLT has low O&D relative to either JFK or ORD - most European carriers serve ORD/JFK and not CLT/PHL for a reason.
Jim