That's been beaten to death by local TV aviation experts of various degree. The best explanation I've heard so far was that it would take them several hours to dump whatever fuel could be dumped and burn whatever couldn't be dumped. By that time it would have been dark in NY. In Warsaw it was a nice day with calm winds, though a bit on the cold side.I still question the decision to fly across the pond without one of the hydraulic systems, but that's just me....
Also, though no one will admit it out loud, the economic aspects might have also influenced the decision. Whatever the nature of the problem, it would be cheaper to fix it in Warsaw.