In the beginning, Barbarossa was launched with the assumption that the Soviet Union would collapse quickly. This did not happen, for various military, economic and political reasons.
Its easy to tell why it happened: HItler got distracted. Hitler pulled resources from the drive on Moscow and diverted them to the attack in the Caucasus for the oil fields. Had he continued the drive on Moscow they would have more than likely made it before winter, Stalin would have had to abandon the city, and most Soviet resistance would have collapsed. HIs insistance on personally controlling the war cost him the war and eventually his life.
I always found it interesting that, in the waning months of the war, many in the German high command clung to the hope that they would ally with the Americans and fight the Russians. Reading histories with 1st hand accounts and personal war diaries of the Wehrmacht can really change how you look at the war and really humanizes them.