The first is logistics. The USSR has the manpower but not the logistical/industrial capability, even after moving key industries. The allies supplied critical war stocks, tanks, planes, ammunition, small arms.
Ummm...USSR outproduced German big time. If wars would be won by production alone German would have been stopped lot sooner. Yeah US helped them but again this only quickened the process.
The second is Leadership. The Stalin purges of the military in the 30's, wiped out the best of the Soviet military leadership. Prior to the war, Stalin was convinced the military was going to overthrow him. So he made those that he thought could oppose him, disappear. By the end of the war, Soviet Military Leadership was strong once again. It had a 5 year college to develop officers better than any Military School is capable of.
Yeah. That's what caused the initial disaster of 1941-1942. Note however that Soviets had the Germans running back before Normandy. And the Normandy was really a side show. Soviets faced triple the amount of divisions than Allies did. And the quality of said divisions was better on eastern front(more SS and panzer divisions for one) so disequality on the two fronts was even wider than just number of divisions shows up.
Stalin slaughtered the officer corp of '30's. He did not prevent military from re-learning lessons AFTER Germans attacked. War is the best teacher of warfare so the soviets were introduced into quite a crash course in "how to win a war". Result which was that the Germans were soon swamped by the superior manpower and production capabilities of the Soviets, halted in their track and then sent backward.
Process which began well BEFORE the Normandy(and indeed which triggered the whole Normandy invasion 'least USSR take German and French under it's wings as well)
Germans couldn't beat USSR whether or not the USA or Brits came. It would just have taken tad longer for the USSR if allies hadn't landed on French(albeit USSR would have then got German+French under their control as well. So probably Stalin would have prefered US+Britain to stay out of Europe despite extra losses to USSR military that would have caused).
Third, the allies tied town literally millions of Axis troops that otherwise would have been fighting on the Eastern front.
Pittances compared to what they faced already. And USSR could take down larger force.
Remember, Stalin was against Operation Torch. He wanted a second European front openned earlier. The invasion of North Africa, Scicly, and then Italy made Stalin downright irritable. I would suggest that if the USSR didn't need the Allies, he would not have cared where we had busied ourselves.
Yeah. No doubt he wanted when it looked bleak but when the inevitable happened and German advance was finally halted by superior manpower+production and after the USSR military had learned required lessons they were already grinding Germans BACK toward German. And THEN the second front opened.
Why you think the Allies landed AFTER USSR got the Germans running? Answer: They wanted to prevent USSR from going all the way to French. They weren't so much interested in helping USSR(if they had wanted to do THAT they would have done when it actually looked like it might actually be needed. Not when it became pretty certain(barring some incredibly unlikely disasters) they won't be needed after all.
Normandy wasn't anti-german manouver. It was anti-USSR manouver. US and Britain knew very well that once German is dealt they will be against USSR's power. Stalin knew that too. Normandy invasion was done with post-Nazi situation in mind.