I’m sure she can act, and perhaps casting actors of a different race than their characters is a bad idea all around (John Wayne as Genghis Kahn!! The role that gave him and a lot of other people cancer), but that misses my point.
A big Hollywood movie will generally want to cast name actors in major roles to draw fans of that actor to see the film. Compared to Ms. Mara, Ms. Kilcher is little known.
I’ll mention strange casting that really gets to me. In some depictions Cleopatra and Hannibal are portrayed as Black (Hannibal looked liked “The Rock” on the History channel). Ancient Egyptians were not Black and Cleopatra was an inbred Greek anyway, it’s entirely mystifying why some people want to insist she was Black. And while there were Blacks in Carthage, it’s very unlikely the upper-class Barca family was among them. Roman busts of Hannibal looked nothing like the actor who played him on the History Channel thing I saw (Bald, Shirtless, Beardless, light skinned Black man).