There's two things I've seen that I don't quite understand:
1 - Why do some people think RDR2 should've won because it's longer? To me, that doesn't make any sense. A longer game != a better game.
2 - Why do some people refer to God of War as a "movie", and thus RDR2 should've won? One of the best things about God of War is the awesome combat gameplay. I haven't played RDR2 yet personally, and correct me if I'm wrong, but from the clips I've seen I'd say that RDR2 is way more "cinematic (/movie-like)" than GoW is.
Predicted 15+ million lifetime-sales for God of War: