The show-makers mentioned the reason they changed it was because fans loved the Sansa character and didn't want to have her doing nothing forever (like she does in the books) or be completely off screen for so long (like Bran). 'Besides, Cogman (a writer-producer) pointed out: “You have this storyline with Ramsay. Do you have one of your leading ladies—who is an incredibly talented actor who we’ve followed for five years and viewers love and adore—do it? Or do you bring in a new character to do it? To me, the question answers itself: You use the character the audience is invested in.” Notice how much more likable Tyrion is in the show? Because fans of the show love him so much the writers made him more likable. I agree with you, though, about Sansa. Can't stand her. But I believe if it had been Jayne, like in the book, way less rage (if any rage) would have occurred among so many people (about a fictional show, BTW).