By all accounts the flights are quite full on the Pakistan-JFK sector, and very few people use the PAK-MAN sectors.
The flight can be done non-stop as its a B777-200LR, however the Americans insist that an extra security search is done, and PK chose MAN for this.
The flight operates JFK-PAK non-stop on the way back, and the Pakistan-Canada route, which also used to stop at MAN, are now direct both ways, as the Canadians do not require extra checks.