It's because BOTW does nothing better than other games have done before. Without the Zelda name it would have gotten slaughtered in reviews.
Eh, disagree. I think the Zelda name buffed it a bit, but it really is a great game. I don't see how the Zelda tag was the difference between it being game of the year and "slaughtered." In terms of pure exploratory freedom and environment interactivity the game is probably unrivaled. The physics system is pretty crazy, and is also to my knowledge unrivaled in open world games. The climbing system gives the game a vertical nature which means 1) there's more to explore per square foot than other open world games and 2) unlike elder scrolls there are no invisible walls.
Also, I think the combat is somewhat better in botw than in w3. I also can't think of another open world action adventure game that's as puzzle oriented as botw is. It legitimately does a number of things that have never been done before or have never been done on this scale, and does nearly all of them well. Have you played it?