Why so difficult understand this?

World War II ended because of the Soviet Union.

What's so difficult to understand about the fact that the most important country in the defeat of Nazism was the Soviet Union? You don't have to agree with the regime or even think it was good, but I thought all history buffs would know this, since we have data on the number of weapons manufactured by each country, and also the number of soldiers killed, not to mention the more subjective data such as the importance of a front to the war.

Is it difficult to understand because of American war propaganda? (At least I thought so until recently because of Hollywood movies). Or is it simply because people don't give due importance to something that goes against the ideology of the majority? I really don't understand.