Why Don't Superhero RPGs Work?

Ok so the title is a generalism because Superhero RPGs actually do work but why aren't they more popular?

Let me explain.  Comic books are going through a bit of a resurgence in recent years.  This is in part due to the success of movies based on Marvel (Captain America, Thor, Iron Man and The Avengers) and DC (Batman, Green Lantern and Superman) properties.  I also think that like Hobby Games the comic book market is experiencing an element of "Geek = Cool" just now.

So why then do I think that Comic Book RPGs don't reach the same level of popularity as say Fantasy or Sci-Fi based RPGs?  I honestly don't know...

It's been one of those "Why?" questions that I've been trying to work out for a long time.  Given the episodic nature of comic books and the opportunity for Superhero team-ups surely this should be a perfect fit?

Maybe it's down to a lack of systems available to play?  Nope that's not it as there's an abundance of RPGs focussing on Superhero type games.

In a previous post I mentioned that I owned but never played Brave New World which was a Superhero RPG using something similar to the Savage Worlds mechanics.  Indeed over the years I've owned a variety of Superhero RPGs but I've never managed to run a decent length storyline and that's speaking as someone who's been reading comics for as long as I've been playing games...

So - Why Don't Superhero RPGs work?