Part of the appeal is the human tendency to believe that the grass on the other side of the fence is greener. I mean, America is the prototype of a non-Communist country, so Communism is the other side of the fence.
Another part is the mere size of the country. Among 250,000,000 people some are bound to be pro-Communist. I don't really see a huge and serious movement toward Communism in the US, so the extreme of the bell curve may just account for it.
Last but not least, every advertising professional knows that if you overadvertise a product, the advertising will work against you. Now, America has gone long ways since Joe McCarthy, thank goodness, but some of it still exists in subtle ways. For example, I am an immigrant. In fact, I am an immigrant here because I escaped from Communist Czechoslovakia. But one of the questions on the application for permanent residency status was: Are you, or have you ever been a member of Communist Party? The answer was easy: Noooooooo!
But five years later, when applying for citizenship, I found the very same question on that application. Then, when I had my citizenship exam, I was asked the same question.
Finally, the great day arrived: I was sitting in the court room, ready to be declared a US citizen. Just before the judge came in, we were all asked to come and sign a paper. Guess what the paper was about: I certify that I am not nor have ever been a member of Communist Party. Now come on, do you really think that in the few days between my exam and the formal citizenship ceremony I had nothing better to do that join the Communist Party?
After being formally declared the citizen of the United States of America a weird thought popped into my mind: I can finally join the Communist Party! Now, I am the last person in the world who would want to do that, I spent the first 29 years of my life under Communism, I was involved in underground activities against it, and I escaped from it. But I keep wondering how the immigrants from countries that did not experience Communism felt about it.