If you read the book "Why the Allies Won," by Richard Overy, you will get a slightly more nuanced response to your question. He argues that it was less due to the larger decisions to take on too many enemies at once and the military might of the combined allies, but had a lot more to do with:
1. The incredibly rapid improvement of the allied armies in both tactical ability as well as technological advancements. The Allies started off so far behind the Wehrmacht but quickly caught up.
2. An actual application of morality, that the combined war effort of the Allies was driven and benefitted from the idea that they were in the moral right.
3. An understanding by the Allies that WWII was a struggle over values and that it had to be won.
Of course, these are just Mr. Overy's opinions, I found the book interesting if not altogether convincing. It is difficult to argue with the factor that the Russians were willing to sacrifice millions of men to grind the German Army to a pulp on the Eastern Front. Still seems pretty big to me!