I would think the number of sales and people who actually bought and played the game would have a somewhat large impact on a game being the "Game of the Year", and not just reviews. It doesn't really matter if a game is great, if it doesn't interest or reach more than five people.
But, then again, they give Oscars and whatnot to art films that are seen by no more than five people every year too, so who knows. I think that the award might as well be called "Game of the Year That Nobody Heard of and Even Fewer People Played". GOtYTNHoaEFPP. Rolls off the tongue.