The simplest explanation is that, especially historically, Hollywood has targeted domestic audiences first. What message do you think is/was more likely to play well in the US and broadly in the west?
Perhaps another thing is that when the memory of the war was still fresh and people (at least in eg France) generally knew that the USSR had a big impact, McCarthyism was also at its height and there were strong reasons not to show communists in a good light. And then maybe later films mostly copied the trend.
It’s like the chase scene at the end of Argo. That never happened. They just got on the plane and left but if the movie was accurate that would be a horribly boring ending so they made up a better on.
The change in this case was not in order to make the story more suspenseful but to simply inspire patriotism.
Any Russian movies about the US winning the WW2? Naturally each country tells their own narrative. This seems like a silly point to make.