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... they're creating a whole universe of movies, which is why Marvel their films are doing so much better than DC's.
Marvel has done a very good job of leveraging their properties into recent films (Spider-Man, Ghost Rider, X-Men, Blade, Fantastic Four). And while not all of them have been good (hello, Daredevil!), Marvel is setting up a shared universe that will allow a lot of crossover potential in the future. (Downey is slated for a cameo in the new Incredible Hulk movie, and there's lots of talk about a possible Avengers flick bringing together characters from Marvel's various movie projects.)
Contrast this with DC, whose movies haven't been managed as well. The fact that Brandon Routh and Christian Bale aren't slated to play Superman and Batman in the upcoming Justice League movie speaks volumes about the differences between Marvel and DC when it comes to their films.