Could easily have been headwinds requiring more fuel than predicted. But, it's crazy that the Captain chose to return to JFK rather than to divert. While he could have diverted to any field which can handle the aircraft, there are plenty of B6 stations along the way.

Only think I can think of is that after a diversion, the crew would not have had time to fly on to SJC, thus better to return to JFK and start out again.