Producer Philippa Boyens explained the decision to make her the protagonist, “Everyone else dies!” And before you start complaining, this is already in the text, and also the story takes place centuries before the movies

Preemptively trying to justify something that she knows people don't want.

Aside from having the film focus on Hammerhand’s daughter rather than Helm Hammerhand, the film also reportedly features a scene that “shows the politics at play in Edoras, with a lord of Rohan wanting to erase the history of a group of female warriors and banner women to the king, a prelude of things to come.”

Yeah so it's straight up feminist fan-fiction. Not that anyone expected anything different.