Answer:
The Soviets took so many casualties largely because of the politics of the Soviet regime. Stalin's purges in the 30s replaced the professional officer corps with officers thought to be more loyal the Stalin and the party (although even the pre-purge officer corps was no match for the Germans).
Step-by-step explanation:
Also, the Germans had much better organization, strategy, and tactics. Soviet troops and junior officers were taught to do nothing without orders; German doctrine was to allow NCOs and junior officers to take the initiative when they see an opportunity.
This added up to tremendously lopsided casualty numbers, where the Soviets would lose 5 to 10 men for every German lost. The only thing that saved them was that they had so much territory to which they could retreat, so many men they could send into the fight, and a steady stream of equipment and supplies coming from the US and UK.