The Red Scare happened because it threatened the American ruling class, and America wasn't occupied by Nazis in WW2 so they didn't have the experience of being liberated by mostly leftist rebels. Immediately after WW2, communists and socialists were seen as liberators who freed various countries from Nazi rule, even in the UK where Winston Churchill lost re-election partly due to him going all-in on anti-communism (which the people didn't like). As a result, Europeans were a lot friendlier to communism and were more open to adopting socialist policies. The US was both the leading capitalist power AND it was very distanced/separated from the oppression and rebellion against the Nazis, so they just saw communists as a threat.