Why were Americans so fearful of communism?

For one thing, there was no real tradition of socialist or left wing politics in America as there had been in Europe. Socialist parties existed, but they received negligible support at the polls. And in time, some of the less radical demands of left wing parties were successfully co-opted by the Democratic Party, particularly during the New Deal. This successfully blunted what little appeal remained for the parties of the Left.
Most Americans were brought up in a social tradition that emphasized what Herbert Hoover called "rugged individualism." Whether mythical or not, there can be little doubt that commitment to hard work, individual enterprise, and the sanctity of private property was widespread, even among those with little or nothing. Communism directly challenged the prevailing belief in the efficiency and benevolence of the capitalist system. It seemed a strange, foreign ideology, one that tended to be associated with immigrant communities, those regarded by large sections of the white Protestant majority as not being "100 percent American."
Another factor in Americans' fear of communism was its avowed atheism. Even today, it is a brave candidate for public office in America who admits to not believing in God. America has always been regarded as "God's country," and so the official atheism and systematic persecution of religious believers in communist countries militated against any serious chance that communism might take root in the hostile political soil of the United States.