One way to approach this question is to examine the reasons that the United States had such a strong reaction to communism in the period following World War II. First though, I shall set the stage with a little background. Communism was hardly new to the United States time. The Communist Party of the United States had existed since 1919. The height of the American Communist party was actually during the 1930s. At this time in the United States it downplayed any revolutionary tendencies and instead communists portrayed the ideology as adopted an American image, building Marx and Engels into the existing American pantheon of heroes such as Jefferson and Lincoln. Earl Browder, the head of the party at this time, stated in reference to this period that "we became almost respectable. Never quite respectable. Almost." Some of the appeal of the Communist Party of the United States was actually its strongly antifascist nature, as fascism was very much seen as a foreign ideology at the time. This changed in 1939 when Stalin signed his non aggression pact with Hitler. The Nazis were seen as a danger by many and antifascism was certainly a popular stance in the United States. The USSR, by signing the nonaggression pact, had tarnished the reputation of communism in the eyes of many and led many liberals in the United States to become skeptical of the communists. The antifascism returned to some degree when the US entered the war and ultimately allied with the USSR, but the party's reputation was still tarnished.

Of course with the defeat of the Nazis in 1945, antifascism was seen as no longer necessary in the United States, as the major fascist powers had been defeated. The war left the world dominated by two superpowers: The United States and the Soviet Union. Communism has often been seen as an international and expansionist movement. The USSR certainly saw it that way at times and for much of its existence it supported communist parties around the world including in nations it did not control. While the US and USSR never fought direct wars, numerous proxy wars were fought in other nations during this period between US and USSR backed forces. Support for such wars may have been justified by the Soviets by the idea of self defense and having a buffer zone between it and nations allied to the US, but to many Americans this was an act of communist aggression. For this reason to be associated with communism after world war II meant that one was not just a little questionable as it may have been seen in the 1930s, it suggested that one was actively working for the primary enemy of the the United States. While Senator McCarthy and his ilk were quite liberal when it came to labeling people as communists, there was in fact some legitimacy to the fear of communism in the United States as the American Communist party had received at least some direction from Soviet Union. While many of the people persecuted by entities like the House Un-American Activities Committee had abandoned the communist party decades previously, there were some communists in the United States who did carry out espionage.

The international campaign of the Soviet Union and polarizing force that the cold war was can be contrasted with the support (or rather lack thereof) for fascism and Nazism in the United States. As mentioned earlier, once Germany was defeated in 1945 there was no longer any major power that embodied those ideologies on the world stage. There were of course still some Nazis and fascists around, but as there was no major nation to back them up their existence would not have been an existential threat in the way that communism was perceived to be and therefore were not hunted down and persecuted by the United States in the ways that suspected communists were.

A number of books have been published on the red scare and American anticommunism, a couple you could look into for more information on the fear of communism are the following:

Ellen Schrecker, Many are the Crimes (1998)

Richard Gid Powers, Not Without Honor (1995)