That may be a partial explanation, but in reading The Anti-Communist Manifestos, I have discovered another, much more substantial reason.
Beginning in 1941, Hitler's Nazi Germany was seen as a juggernaut, and the world felt that only the Soviet Army was strong enough to stop it. In the mid-1930's, says John V. Fleming in his book The Anti-Communist Manifestos:
The nearly universal concern of American opinion makers of the...1930's...was Hitler's Germany. The great enemy was Fascism. There were three regimes in the Fascist camp--Italy, Germany, and Franco's Spain.. The great enemy of Fascism, and many thought the only committed and adequate one, was the Soviet Union.
Roosevelt and Churchill
realized[felt] that the only force that could defeat Hitler was the Red Army. (pp. 111-113)
In 1938 and 1939, that began to change. The absurdities of Stalin's Soviet show trials began to come to light. Then, to the surprise of nearly everyone, Stalin joined forces with Hitler.
From the beginning of September 1939 to the middle of June 1941--the period of the start of the war in Euope, of Hitler's sensational conquest of France, his frustration in Britain--the Soviet Union was the ally of Nazi Germany, busily gobbling up half of Poland and as much of Finland as [possible].
Embarrassed, Western Communists went largely into hiding. But then a stroke of good luck for them ensued. Hitler attacked the Soviet Union.
All of this changed...with dramatic suddenness, when Hitler unleashed his invasion of the Soviet Union in June 1941. Now Western Communists rushed to encourage a United Front in defense of democracy, civilization, and the Soviet Union. Almost overnight, American anti-communism became something rather more than inexpedient or undiplomatic--it became unpatriotic. With Hitler's invasion of Russia in June, and the resurfacing of the American communist officials as superpatriot cheerleaders for a United Front, things changed abruptly.
That is how Hitler became the goat of the 20th century. And that, unfortunately, is how America came to give Stalin the Butcher a pass.
And to think that American bankers provided the the cover that, in one case, brought one of these megalomaniacs (Hitler) to power and, in the other, ensured that the other (Stalin) would remain in power. In retrospect, both leaders were mass murderers, and from that perspective, neither is more evil than the other. From sheer numbers of homicides, however, Stalin makes Hitler look like a school girl by comparison.
But you don't learn that in school. How unfortunate.