Because I had occasion to talk to a Brave board artist who tells a story a wee bit different than this:
After [Brenda's] version of "Brave" proved too complicated, too dark, and too, er, snowy for Pixar, she was removed from the film and the studio's vaulted Brain Trust collective, and replaced by storyboard artist Mark Andrews, who simplified things and (at the urging of the Scottish Tourism Board) made the Scottish setting lush and green. ...
The board artist says:
"I was on the picture for years. The picture as released is still Brenda's story. Some of the mechanics are different, and some sequences are changed around, but its core is still what Brenda created." ...
So why did Ms. Chapman get pushed out of the director's chair? An artist who has known her a long time said: "I wasn't up there, but Brenda is vocal, and defends her artistic vision. She probably didn't want to change things they wanted changed, and said so. And it didn't go down well."
The general rule of thumb in Movieland, now and forever, is: If the boss wants to go North, and you want to go South, you better change your mind or sense of direction, because otherwise you will not be working on the project for very long. But as I heard Brenda herself say several years ago:
Pixar is a boys' club. I'm the token female."
Well, she's a token no longer. She now works for Lucasfilm, headed up by Kathleen Kennedy.