Communism

Why do people in the U.S. think communism is so horrible?

Because it has been illegal. Other countries where it has not has enjoyed multi party democracy and freedom of speech and a lower level of repression of counter establishment views. It does not matter what you think of communism, the fact that the US hasn't got a real (as in relatively big and respected) socialist party is a democratic failure that is the effect of government oppression. So enjoy your tyranny.