The key thing to understand is that Marvel is CHEAP thrifty and they would rather give opportunities to obviously talented but underused directors and actors than spend a fortune on someone that everyone knows is good.
Plus they have a pre-existing relationship with Joss on Runaways and Astonishing X-Men.
A lot of what Marvel is doing is convincing people to do their films for less (and signing them to multiple film deals) because being in a "cool" comic-book movie that makes a ton of dough will help their careers.
Saw it tonight and loved it. The digital photography was just stunningly gorgeous. The story was predictable, since everyone knows what happened to John Dillinger, but it was well-acted and even suspenseful in spite of the known ending.