Yes, this exactly. The CCA grew out of a widespread moral panic about comic book content, and its effects on youth. See http://en.wikipedia.org/wiki/Seduction_of_the_Innocent. Basically, after this, the once thriving comics industry in the USA was reduced to just superheroes as all other genres like romance, horror, detectives (there's a reason Batman was published in detective comics), science fiction, were considered too "dangerous." It's quite unfortunate.

There's other reasons of course as well - notably the rise of the collectors market, but the USA today is still overwhelmingly dominated by superheroes in the comics market.