Before I give my views on the subject, I’ll stress that I agree with you, Daoloth. I think that National Socialism and Communism are two sides of the same coin. Both will happily shoot you in the head, blow up your synagogue, or ship your entire family off to some wasteland to make their points. When you get past theories into practice, Communists show precious little difference from Nazis.
I’ll also stress that I’m no McCarthyite. I think that communism was a danger earlier in the century, but I don’t think that organizing witch hunts or dragging people in front of congress for exercising their constitutionally protected right to express opinions was the answer.
As for why communism is more highly regarded? Well, they have a pretty good message: Everyone should be equal, and the labourer forms the bedrock of society. Hell, I agree with that. Equality is a noble goal to shoot for, especially when your country is ostensibly built on the notion of equal rights for all.
In contrast, the whole “master race” message of the Nazis gave out seriously bad vibes to anyone with a two- or three-digit IQ that wasn’t a nutjob. Nazis just had “bad” written all over them.
I think someone could conceivably join the communists in a sincere hope for a better world, however deluded that hope might be. No one joins the Nazis, however, unless they hate at the outset. The Nazi message is very clear. If you are not caucasian, you DO NOT deserve to live. It’s a bad system with bad PR. The communists have a bad system with good PR, and that makes a big difference.
I’ll end this with just a couple of disagreements I have with your OP. Trotsky and Lenin advocated violence. Were they terrorists? Doubtful. I’d say there’s a fine line between revolutionaries and terrorists, but these guys didn’t really target the civilian population the way OBL does.
Also, the facists didn’t kill less people because they were any better than the communists. They simply didn’t have the time, because their system was overthrown much earlier than the communist system. I understand that this wasn’t the original intent of your sentence, but some people could read it that way.