In my opinion, the Allies defeated Germany by having more people to fight with and a much larger industrial base to produce war materiel. Very significantly, the Allies had the United States to serve as a huge factory that could not be touched by the enemy the way German factories could be bombed.
When you combine this industrial advantage with the fact that Germany faced both the US and the Soviet Union (with huge populations -- I'm not discounting Britain, but it's not a huge country) it is not at all surprising that the Allies won the war.
There are a number of answers to this and a number of books written about the subject as you will find huge variances in the arguments of various historians.
One view, that I tend to agree with, is that the main reason was the utter destruction of the German army on the Eastern Front and the willingness of the Russians to sacrifice huge numbers of people and massive amounts of material to bring about this destruction.
In many ways the invasion of Normandy and the other campaigns in Western Europe were only possible because the might of the German Army had been absolutely wrecked on the Eastern Front. Had Hitler been able to bring the same forces to bear in the West and not been so obsessed with trying to take Stalingrad, etc., the war may have gone very differently.