What were the reasons for the Allied victory in WWII?
The Allies were able to win WWII for two major reasons. These were first, the size of the Soviet Union's land mass and second, the size and secure location of the United States' manufacturing sector.
The Allies relied heavily on the United States' ability to manufacture huge amounts of war material. This ability existed because of the richness of the United States and because of the fact that it was located in a place that could not be easily invaded or even bombed. This allowed its factories (unlike those of Germany, for example) to work continuously throughout the war. The products of these factories eventually overwhelmed the Axis.
Secondly, the physical size of the USSR helped the Allies win the war. The German invasion of the Soviet Union ended up leading to the loss of huge numbers of men and weapons. This happened even though the Germans were clearly superior to the Soviet Army. What made the difference was the fact that the Soviets could use the size of their country to overstretch the German invaders.
So, the Allies won because A) the Germans exhausted themselves trying to conquer the huge area of the Soviet Union and B) the US was able to churn out huge amounts of war materiel to be used against the Axis.