I doubt if the cold killed them. There had to be another reason, starvation, Mites etc.. that got the best of them WHILE they tried to survive the cold. If they had honey, but did not move to it because of the cold, then that is at least partially genetic. (IMHO)
We have had temps WAY below normal, A couple times with wind chills of -50 F.
Moisture raining down on them will do it, but it sounds like you did everything right wintering them. If you had foam on TOP, and an upper vent, then the condensation should not have killed them, so there should be another explanation lurking among their little bodies.
Were they stuck in the cells head first when you opened the hives?