And they were also essentially fighting a completely different war. The War in the East and the War in the West were two completely different things. You have to remember that when the Nazis invaded the USSR, their enemy wasn't the USSR, their enemy was the people living in the USSR. It was a struggle of such massive scale that of course war crimes committed by the Allied armies on that front were larger than those an the other fronts. And even by that logic, the USSR didn't set up extermination camps for tens of millions of people.
And I know you're going to say "but teh gulag!", and it's bullshit: