Because the how/why of getting her powers requires a lot of set-up. Act One of a film is for establishing your characters/setting, before ending with an inciting incident. If you include in that 1st act a Kree invasion and a whole bunch of circumstances that get Carol next to an exploding Kree device, that would give us like zero time to actually get to know Carol in her natural habitat and her backstory or develop her relationship with Mar-Vell before we change her life/circumstances forever.
Why does it need to require any more set up than Steve Rogers becoming Captain America did?
Captain America The First Avenger wasn't about the super soldier serum that gave Steve Rogers a six pack. It wasn't about the scientist that created it. It also wasn't about Bucky Barns or Peggy Carter or Howard Stark. A Carol Danvers Captain Marvel movie would also not really be about the Kree, or Mar-Vell, or the Psyche-Magnetron that gave her her powers. So why would you need to devote any more screen time to these things than is absolutely necessary to further Carol's story?
If Black Panther's getting introduced and established 2 years early in CW (not to mention the seeds being planted for his world in AoU), with the relatively cut-and-dry origin of his powers/significance, I see no reason Carol shouldn't get that kind of time to organically establish her place in the MCU as well. Saving it all for the one movie really hampers and cuts down her story/development, imo.
Are we going to get Black Panther's origin in Civil War? I sure hope not, because that would be just as dumb as shoehorning Carol Danvers origin into Avengers Infinity War part 1. I don't see how you would be able to do justice to that backstory in a Civil War movie.
So let me get this straight. You think putting Carol Danvers superhero origin in a CAROL DANVERS CAPTAIN MARVEL MOVIE (where she is like... you know... the main character and stuff) "hampers and cuts down her story/development"... but yet shoving it into another character's movie (that is not even about her) is perfectly fine for her character development...
...ummm okay...
In what world does that make any sense?
As for Black Panther in Civil War; he is going to have Captain America and Iron Man trying to get him on their side because his country controls a rare and powerful metal. That introduction makes him a big time player in the MCU. Introducing him in Civil War in that manner works awesomely well for him as a character. You can do that with him. That's fine. That works.
You cannot do that with Carol Danvers the Air Force Major. Nor Carol Danvers the head of security at NASA. Because what kind of intro can you give her in that role as a supporting character that doesn't just make her another Maria Hill or Sharon Carter? What kind of impact could she possibly make in that kind of a role? Certainly not the kind Black Panther is going to likely have on Civil War as a supporting character, so it would just be a waste of an introduction.
That is why that comparison really doesn't work.
She was a character in the Marvel Universe well before she became a superhero, and had her accident in another story before getting her own title. I see no reason the same can't happen in the MCU.
That's because she was never originally intended to be a superhero. She only became one because they trademarked the name Ms. Marvel and needed a female character to be called that they could keep it. She was also a supporting character to Captain Mar-Vell. That is ultimately why her origin as a superhero took place in his book.
But how the comics did it originally doesn't work in this case, because that character isn't going to be the star of the Captain Marvel movie and isn't going to be a main character in the MCU even if or when he gets introduced. So why make her a supporting character to a man that is ultimately going to be a supporting character himself in this universe?
That just diminishes her importance, and gets her introduction into the MCU off on the wrong foot. That is not going to do her any favors when you ask people to pay money to watch a movie in the theater that features her as the main character.
A large part of the reason I feel this way is because I don't want to see just another origin story. We've gotten enough of those, and the "ho-hum" reactions to Ant-Man's trailers have really hit home with me how antiquated that formula feels at this point in the golden age of superhero movies. If we get the majority of that pesky origin out of the way in a different movie, we can focus on a movie largely with Carol AS Captain Marvel, having Captain Marvel-y problems instead of the nuts and bolts of how this happened to her.
You don't want "just another origin story" but you do want to have a ****** shoehorned version of it in an Avengers movie that is supposed to be about Thanos and the Infinity Stones...
...ummm yeah no let's NOT do that.
Because if you do that, you are just going to short change both her story and that Avengers movie and what story it needs to tell and what it needs to set up for it's story in part 2. That is not fair to either one.
And I liked the second Ant-Man trailer, but I am not expecting an ANT-MAN movie to be a great all time classic comic book movie regardless of how it wants to tell it's story. Because it's Ant-Man for **** sake.