Why was Stalingrad such a disaster for the Germans?

Their entire 6th Army was destroyed. An entire sector of their front line gone. Plus all the equipment that could not be replaced, tanks, trucks, guns, planes. It was a tactical and strategic disaster. Also, you can't look at how many troops it was compared to the initial invasion force. Look at their strength at the time of the battle. Hitler had forces diverted from the eastern front to Italy in response to the allied invasion there.