But really it's because there was a deliberate and concerted effort to erase the USSR's role in defeating the Nazis in western europe, after say 1960. Through public education, and general coverage in medias/fil/documentaries. The US cultural hegemony is real :)
It started earlier, but at the time when the Communist Party was at 30% in a lot of countries and 90% of the workers where in unions the disinformation worked very poorly.