Question

Why was House Un-American Activities Committee especially concerned about communism in Hollywood?

Answer 1

One reason was because they were concerned that Communist propaganda could find its way into film for everyone to see. A film may look innocent on the surface, one could say, but hint at Soviet sympathy with a particular character or a line of dialog. It was an extreme form of censorship and suspicion that ran wild during this period of the Red Scare.

Imagine that your favorite film was accused of promoting something "un-American" even though it may have only been one scene with an angry character or someone making a big speech. It, and those who made it, could then be questioned for why that was so. We laugh at it today with so many movies featuring characters doing things that are clearly un-American, but back then, moviemakers could end up blacklisted or worse for merely being suspected of corrupting the minds of those watching it.

Answer 2

Nazi Germany had just proven the remarkable power of propaganda and broadcast media to keep an entire nation of people disconnected from reality. Even today, there are restrictions on TV an radio in Germany to prevent the possibility of such a thing. (The United States has the First Amendment, which is usually considered sufficiently protective in that regard.)

Additionally, they had just experienced the considerable damage that communist infiltration could achieve, with the relevations of wartime spying by the Soviet Union in the US atomic-bomb project, which led directly to the acquisition by the USSR of nuclear weapons in 1949 -- about 20 years earlier than anyone had thought possible.

Combing the anxiety provoked by nukes in the hands of Joe Stalin with the awareness of how easily it had been for Hitler to hold the German people enthralled, by the clever use of broadcast media and propaganda, and finally the pure fact that Hollywood, then as now, had stronger sympathies for socialism and communism than the typical American, and you have the perfect storm of events that led to the HUAC's suspicious focus on Hollywood.