In addition to what /u/Eskabahh said, I would also add that Dark Souls 3, even though it caters to a more casual audience, isn't really a game that the general audience of gamers would enjoy for a long period of time.

There's practically no actual story (no, Lore is not the same thing as a story), replay value is extremely low unless you enjoy the PvP mechanics of the game (which is already difficult for some as the mechanics are still being polished up 8 months into the game's life cycle) and there were quite a few graphical glitches at launch that didn't get patched until two weeks ago (and even now more and more players are reporting errors, crashes and significant FPS dropping).

The biggest thing, though, is that as a standalone game, disregarding the prequels, Dark Souls 3 cannot stand on its own two feet. This is a bit hard to describe, but (as basically as I can put it) the game lacks the individuality to be called a "new" game. It's not "Dark Souls 3" more than it's "The not-quite-as-good Dark Souls." This is a major issue with making sequels that game developers really have to be careful of if they're trying to be the best game of the year (which I don't think Miyazake was actively aiming for, but who knows).

The Witcher 3 won GotY last year because even though it was a sequel the game could stand on its own two feet and presented a unique gameplay experience to players while maintaining its presence as a Witcher installment. Games such as The Witcher 3 are games worthy of GotY. Dark Souls 3, as unfortunate as it is, falls just short of this benchmark.