I've been long perplexed by that very question myself, and I like the possible answers you pose. Communism seems a very seductive ideology to academics. Another reason I might add is that it made inroads into western societies during the economic troubles of the 20th century. There was a genuine loss of faith in our capitalist system which leftists exploited. There were also many in politics and communications who deliberately covered communism's backside. To this day it maddens me how villains in movies etc. are still frequently Nazi's, but almost never communists.
It got a pass because most of the mass murders ordered by by Mao, Stalin, Pol Pot, et al, occurred long after communism was already a popular brand with America's "intellectual" elite. Even by the '40s, communists and their sympathizers were well-imbedded in the State Department and in major media outlets (including TIME Magazine and the New York Times) here in the United States.
Acknowledging communist atrocities as they became revealed during the '50s to the '70s would have simply been too embarrassing for these people. The mass murders of millions made communism look bad, you see. Couldn't have that.
FRegards,
LH
And a lot of leftists actually do acknowledge Communism’s death toll, but rationalize it with “You have to break some eggs to get an omelette.”