The game raised the bar on open world games, by a large margin when most thought it wouldn't. It is huge, full, and non repetitive for the most part with unique puzzles and complete freedom.
There are other worthy nominees sure, but to deny that Breath of the Wild does not have the odds on its side to win GotY is just denial. Even Red Dead 2 will have a hard time making it's mark against it if it releases this year.