Unfortunately Hollywood casts people that they think will sell the film, not the actor that is best suited to the part.
Hollywood is run by the money men nowadays,not by guys that have any artistic ideas(sadly) as in the past.
There are probably many more suitable actressess to fill the part,but all the money men can see is bums on seats,regardless of how bad the chosen person will be.
This is why some eargerly awaited films of books fall flat,the casting just doesn't fit the character.