Because she went through absolutely no character development. The change of heart is so jarring that it's silly. Why at the end? Why not sometime during the journey? I lugged her around the whole time not caring about her, and when she leaves me she tries to pull a heartstring. It just didn't work because I never liked her as a character. If they at least hinted it once after the first half of the game that she was gaining some sort of emotion, it would have made all the difference, imo.