Originally Posted by Airwings
Movie stars belonged to block busters (people wouldn't have gone to see block busters with unknown actors in the lead).
Then add something else to the equation, and we have the logic answer already there:
Superhero=expensive to make
If Hollywood was the same way today, then Cruise, Pitt, Depp , McGregor, DiCaprio, Damon and others would have been cast as A-list comic book characters. But something changed over the decades. Now only half of roles in the genre are played by stars, and it's mostly supporting characters (Marlon Brando, Nicole Kidman, Kevin Costner, Samuel L Jackson, Natalie Portman, several in Nolan's trilogy etc) or villains (Gene Hackman, Jack Nicholson, Michelle Pfeiffer, Willem Dafoe, Nick Nolte [sort of], Kevin Spacey, Jeff Bridges etc).
Edward Norton, Ben Affleck and Nicholas Cage are the only exceptions I can think of right now