Oh boy, I just read a book about this! The Myth of the Eastern Front: The Nazi-Soviet War in American Popular Culture does an excellent (and very readable) job of going into the creation of the "myth of the clean Wehrmacht".

The short version is that a horde of barbaric, Asiatic Russians raping innocent German women fit the post-WWII American narrative much better than a bunch of barbaric, white Germans raping innocent Russian women. At the end of WWII, the Russians very quickly became our enemies, while the (west) Germans transformed into our obedient client, and then eventually our fully fledged allies. Germany was and is culturally closer to America than Russia, and the average American found the Germans to be subservient and cooperative, especially compared to the Russians. As more and more Americans who had never fought the Germans were stationed in Germany, they formed a receptive audience for horror stories of Russian brutality, which fit into both the political tensions of the Cold War and the preexisting racist assumptions of the character of the Russian people as a barbaric, uncontrolled, and overall uncivilized. American soldiers and officers relied on the experiences of former Wehrmacht officers to inform their strategy for a hypothetical future fight against the Red Army, and this professional relationship proved another vector for the transmission of these ideas.

By contrast, there was little to no opportunity for fraternization with Soviet soldiers, much less Soviet civilians, which gave little opportunity for the transmission of stories of German atrocities. Americans were informed of the horrors of the Holocaust, of course, but it was difficult for Americans to associate those stories with their good friend Franz, who seemed like an all around decent guy. It became much easier to believe in a "clean Wehrmacht", that had apolitically and honorably fought whoever the Nazis had pointed them at, rather than the politically indoctrinated, enthusiastic and bloodthirsty force that had actually fought Hitler's wars. There were no such compunctions about portraying the Russians in the worst light possible, which has lead to the current state of affairs.