Well I gues you could say that the US is the ultimate Capitalist country: Democratic, anyone free to start their own business, large corporations, little state interference, low tax, get as rich as you can.
And I guess you could say Communism is the opposite end of the scale: No elections, everything state owned, little chance to start your own business, no chance to get rich, high state interferance.
So they saw the rise of communism as a threat to their way of life so felt they had to fight it.