Because the universe, time, history etc - everything is predetermined. There is no free will. There is no, one might argue, consciousness either, since no choices are being made and everybody is just a puppet in a show that's been written long ago.. or somewhere in the future.. well, it's irrelevant when. That's the big question for me: did she really make that choice ? Was it hers to make ?
I think the knowledge of the language alone made the change, though I'd say this visit to the ship accelerated this process and Louise understood what is going on instantly, when to others it probably would take years of exposure to the language for change to appear.