Why did superheroes win?

At least part of the slump was due to the end of the war, which had provided so many easy storylines (look out, it's Nazi terrorists, let's get them; Marvel/Timely's superheroes were especially war-based) and possibly a greater public interest in escapism (which, if westerns and romances were the top genres after that, is sensible: those are the two most stable genres in American fiction for the twentieth century, rarely the most popular but always consistently selling). The Wertham stuff hit it a little, but I don't know if the sales impact of that on superhero comics was really significant -- the hearings came when the superhero lines had already been pared down.

Possibly another part of that answer could include "as a result of the superhero genre being the only one born in comics, it's uniquely suited to maximizing comics' storytelling potential and suffers the least loss from adaptation to a medium without the motion of film or the text of prose." I'm not sure I believe that.

I definitely believe that many of the other genres have stuck around by hiding themselves inside ostensibly superhero stories -- there are types of science fiction surviving in Dr Strange, Green Lantern, the Fourth World, and Silver Surfer which have not survived in the novel- and short-story-driven science fiction industry, and a great deal of the dramatic tension in a lot of comics is romance-driven -- and that the shared universe conceit is a significant aspect of the superhero genre's success, one which probably helped the Lovecraft/Ashton Smith/Derleth/etc stuff of the 20s but wasn't exploited by many other genres contemporary to superheroes (and in some cases, couldn't be to any degree; what use is a shared universe to the romance genre, and how would we notice?), because it's a brand name that refers to something that is tangible outside of the contents of the story itself -- but I'm going out of town in about ten minutes, so I'll see if this thread's still alive next week.

(eg there are also war MOVIES)

(I'm also curious about how well erotic comics sell.)

Well, no one else did it first; and the genre is rarely done in another medium except as adaptation or parody, which is fairly remarkable -- especially for a genre that's been popular for this long and whose biggest success stories have made so much money.

Doc Savage, the Shadow, etc., are pulp heroes/mystery men/whatever you like -- forebears of superheroes but distinctly not actual superheroes (although you could argue they've been used like superheroes in a lot of their revivals).