Oh boy, Finns v. Russians. My money’s on the Finns!
As to the OP, the reason we didn’t do anything about Stalin while we did Hitler is, for an American, easy. Hitler’s Germany declared was on the United States of America on December 11, 1941. Stalin’s USSR did not declare war on us. Hence we fought the country that declared war on us.
As for the Blitzkrieg into the USSR, it didn’t fail, not at all. It succeeded in what it set out to do: destroy the Red Army. Rather it destroyed what German intelligence estimates had said was the strength of the RKKA. Those estimates were way under reality however.
Even so, the losses taken by the Soviets in the first four months of fighting should have knocked them out of the game. Hitler’s “whole rotten structure will crumble” idea. But it didn’t. The level of control of the people by the state was also underestimated by the Germans. As was the tenacity of the Soviet soldier when properly lead and equipped. So the structure didn’t collapse.
Anyway, blitzkrieg then (and now), isn’t about territory. It’s about destroying the enemy’s ability and will to fight with concentric attacks and deprivation of supply and communication. The move south by Army Group Center’s mobile forces south to trap and destroy the Soviets around Kiev was completely in keeping with the blitzkrieg idea. The original Barbarrossa clearly states that the goal is destroying the Red Army. Hitler himself was of the view that territory, including taking Moscow “is of no great importance.”