: Hello,  
 : I don't have time to properly include this in the article, but here are a few answers, from different source as this matter have been heavily discussed by historians: 
 : - Germans overestimated their forces, strategically as they didn't have the means of their ambitious case blue plan and tactically as so far they had rather easily repulsed every soviet offensive   
 : - the German intelligence services failed terribly. They misled the high command in thinking the soviets were strong enough to lauch 2 major offensives (Mars and Saturn) at the same time, hence Russian preparation on the Don river were expected to be done for local attacks and not for a strategical scale offensive. 
 : - the German plan case blue, which led to this overextension of the flancs, was a kind of a gambit  it was supposed to give such a powerfull blow to the Red Army that it wouldn't be able to continue the war, so overextended flancs didn't really matter in front of a dying opponent.  
 : - Operation Uranus was a grand premiere and even the Russian command was unsure of its results until the end (I'd say until the end of operation Wintergewitter in the end of december). 
 : - Russians used military deception (maskirovka) to the fullest, transmitting major orders only verbaly, creating false radio traffic, moving only at night etc... hence the 5th tank army and the Don front were only detected at the very last moment, way to late for the Germans to react. 
 : - 6th army had a chance to escape encirclement by abandoning Stalingrad arround the 21-22 december (before the junction of the soviet armies). Paulus and all his senior officers have issued such a proposal, but this was refused by only 2 key characters  Hitler and von Manstein that was Army Group Don commander and Paulus direct superior. 
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 : I am sure that we can find many other reasons, it's easy to judge when we know the end of the story, but things are always blurry behind the fog of war. 
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