Attacking Stalingrad instead of going after the oil fields was a biggie
The truth is that Hitler was out generaled. He failed to grasp the depth of the Soviet army and attacked with an inadequate force.
Cold and clothes were inconsequential to the defeat.
I think that attacking Stalingrad was almost a military necessity. It would have been extremely dangerous to leave a military center to your rear as you advance deep into the Caucuses. OTOH, if you could take Stalingrad you could hold it with far fewer troops than it would take to cordon the area, like a seige.