There are really three ansers to that.
Most Americans value three things: The right to free speech, the right to property, and the right to freedom of religion. Communism goes against these things. In Marxist teaching, the ideal society would contain no religion, and no individual property. Thus Americans fear communism.
There are many reasons why America was so scared of Communism:
after WW1 is when most Americans were afraid it would spread to their own soil and it got worse after WW2, so some might argue it went on until the Cold War was over. some people are still scared of all types of communism today.
Americans Battling Communism was created in 1947.
Americans were very worried about the threat of Communism in the 1950s. This fear was so prevalent at the time that the 1950s are often called the time of the "Red Scare."
In part this was because of one of the main themes within the basis of communism: that communism must spread to the rest of the world. This would leave the Americans with no-one to trade with and their economy would collapse! Also (maybe more importantly?) communism opposes/condemns capitalism and America is a capitalist nation. You see the issue.
They were afraid of the spread of communism.
Communism and the Red Scare
"Let the ruling classes tremble at a Communistic revolution." Because the ruling classes in America during the cold war trembled in fear of a communist revolution, and unfortunately were successfully in conditioning people into fearing and hating communism, while most Americans to this day do not even know the actual definitions of socialism, communism or Marxism.