Answer:
Numerous factors, which I don’t have time to do more than just list:
The USA never (officially) had a severely repressed peasant class and a group of nobles keeping them down.
The USA has a longstanding culture of individualism that doesn’t support people getting together for organized political action as well as other countries do.
The government suppressed Communist and proto-Communist organizations pretty harshly.
The government also took the wind out of their sails by actually making some reforms and doing some of the things they wanted: votes for women, labor laws, workplace safety laws, unemployment insurance, Social Security, Medicare, and so on. These weren’t all done out of the goodness the government’s heart, and business screamed its head off as it always does; but they had the effect of reducing interest in Communism.
The US has a huge culture of entrepreneurship. Many European countries do not. In Europe, corporations are things founded by plutocrats, not by ordinary people, but in America, most people can envision themselves running a small business even if they never intend to. This, too, discourages interest in Communism. As John Steinbeck put it, “I guess the trouble was that we didn't have any self-admitted proletarians. Everyone was a temporarily embarrassed capitalist. Maybe the Communists so closely questioned by the investigation committees were a danger to America, but the ones I knew—at least they claimed to be Communists—couldn't have disrupted a Sunday-school picnic. Besides they were too busy fighting among themselves.”
