Kevin Feige addressed what happened in an interview around Avengers: Age of Ultron’s release.
The Marvel boss said: “There were drafts that maybe people somehow got their hands on and read early on that included some characters – maybe others, but certainly one, which is why it’s dangerous to read scripts early on, and talk about them early on…
“[Captain Marvel] was in a draft. But to me, it would have done that character a disservice, to meet her fully formed, in a costume and part of the Avengers already when 99% of the audience would go, ‘Who is that?’
“It’s just not the way we’ve done it before.”
source: express.co.uk