It's pretty simple: Marvel is NOT going to pick an unknown for their first female-led film.
Look at it from the studio's perspective: you have $50M+ funding to make this a hit; you're not taking any chances to pick a chick who looks the role but can't carry a film or has no prior hype/credentials.
Marvel could've taken chances back then with Hemsworth, but now they cannot afford to nor will they put themselves in the position to take an L. I wonder why the logic doesn't apply to Cumberbatch, Boseman, and whoever else got cast as leads in Phase 3. Even the kid who got cast as Spiderman has a solid background. Marvel has rarely, if ever, done wrong with casting.
Why should CM be any different?
Edited by Slighted - 8/1/15 at 10:48am