Why Hollywood isn't seeking Americans to portray superheroes
My guess is that it's because, unlike the UK's Mark Millar, Grant Morrison and Warren Ellis, a writer from some of these aforementioned countries wouldn't have the same bleak, downbeat approach they could, laced with hyperviolence. It's quite possible that some Australian based writers with potential could indeed wish to take a more optimistic approach than what the British ones do, and TPTB wouldn't allow it, discouraging them from ever seeking a comics writing gig with DC and Marvel.
And on top of all that, the truth is that foreign based writers from outside the American continent aren't being given a chance to tackle American comics, unlike American movies, where they could be. Which could also provide another clue to how the comics medium isn't very "diverse".
Going back to the subject of movies, it looks as though the upcoming Superman film may not have Lois Lane in it, similar to how the Spider-Man reboot won't have Mary Jane Watson.