Please understand that to people who don't believe in Communism, this may sound an awful lot like a suicide pact. "For this work, we are all going to have to jump in." And according to Marxist theory, as evidenced by its dependency on internationalism, it requires that at least many of the world powers adopt this thinking all at the same time and with the same goals in mind. In a lot of ways, it comes down to Game Theory. In the Prisoner's Dilemna, it always benefits the individual to defect. In terms of nation-states, defecting on Communism can either be not signing up to go down with the Red ship, or it could be a way to become a global power as everyone else signs on board. Getting all these people to believe the same things at the same time, working towards the same goal, seems far too hopeful.
I agree that communal societies can and have worked. To debate this would be to deny history. The Native Hawaiians even adopted these kinds of ideas. But what inevitably happens in an increasingly globalized economy is the influx of new ideas, new motivations, and new sources of competition. Maybe it's the invention of the gun into your world, maybe it's a car, maybe it's just the prospect of having more time for leisure. Whatever it is, something will invariably trigger people to defect and become selfish. What I appreciate about capitalism is that it attempts to harness that natural desire that we all have, accepting the good of their invention and labor as a byproduct that aids society.
As for the spread of Communism to the US, I doubt it would have happened. The Socialists kind of had their chance to make it stick in the US, and they were at their strongest up until about the 1920's. As evidenced by voting results in national elections, the Socialists were losing popularity prior to WWII, and afterwards, they had no hope. It doesn't help that Stalin was doing his Great Purges, killing millions of people in the Gulags. The number is not known, but from what I can tell, the average estimate is about as many as Hitler killed in the Holocaust: 11 million people. It's going to be really hard looking like the good guy after something like that. Had he been a more benevolent leader, and had the Communists been more benevolent overall throughout Eastern Europe (not sure that this would have been possible for them, as totalitarianism is almost always cruel in its execution). In short, I think that the idea of Communism is nice, but it is impossible to execute in a large group and over a long period of time. Unfortunately, the results of when it goes wrong are horrifying for the people subjected to it.