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.
I recently got a Blockbuster gift card for my birthday, and parlayed it right into one of their Movie Freedom Passes, the one that you get 2 movies at a time for a month, any movies for however long. The first month you try this, apparently, its only $14....