Read up on the history of the Comics Code Authority[1]; I suspect that after the Comics Code was introduced, the remaining comic publishers were over-eager to prove their products were the exact opposite of the content prohibited by the Code, setting the thematic pendulum forever swinging toward and away from that content, instead of exploring broader themes.
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.
[1]: https://en.wikipedia.org/wiki/Comics_Code_Authority
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