At the root of it, it was pilot error. Yes, the pitot tubes froze over giving inaccurate readings but it was still very possible for the pilots to maintain control of the aircraft. However, due to ignoring the stall warning, keeping the stick held back, failing to cooperate and general panicking they crashed the damn thing.
This - when the most experienced pilot came back into the cockpit he seems to have realised almost immediately what the two co-pilots were doing wrong and called out the correct action to take. But he was too late.