Uh... heard of something called
Operation Uranus ? The city was just a bait and a trap to tie the Germans down, since thanks to Adolf's shining genius they were supposed to take the damn thing pretty much no matter the cost despite its rather meager strategic importance (especially as a jungle of blasted ruins). Leave it to crackpot dictators to make really stupid things a prestige issue...
The Soviets, realizing this, were only too happy to keep the Germans there not in the least because the confines of urban warfare negated just about all the advantages the Germans had, while
they got ready to demonstrate they too could do that kewl Blitzkrieg thang.