For what reasons did the Allied powers finally defeat Nazi Germany in Europe?

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.