He was not the main reason the Nazis lost. Just one of many.
Three other reasons trump Stalin…
1) the U.S. kept the USSR supplied with everything from trucks to SPAM. Without our material help, Russia might well have lost.
2) the moment Germany declared war against the U.S., Germanies fate was sealed. Our primary focus would have been on Japan, since it was Japan that attacked us, not Germany. Sure, the U.S. would have eventually fought Hitler, but our national anger was directed at Japan, at first.
3) when the U.S. sent our Air Corp to England in ‘42 and Operation Torch was carried out in North Africa in’42, and subsequent operations in Italy and France forced Hitler to shift forces to those theatres.
In conclusion, it took all of us to defeat Hitler.