There are around 2300 SFWA members, mostly published authors/artists, with some editors and other industry professionals. The nomination process is that any of us can send in a slate of up to five works in each award category that we think should be up for consideration, our votes get aggregated, and the five works with the most votes are the nominees. I’ve got no idea how many members send in a ballot. I always do, and clearly a lot of other people do too, given that the nominations rarely reflect my preferences. 
Anyone speculating about “backdoors,” as some folks were upthread, is groundlessly imagining a much more controlled process with fewer people involved, and perhaps one more susceptible to publisher influence. The Nebulas reflect what a majority of authors like to read/watch/play.
And maybe it shouldn’t be a surprise that a couple of thousand authors voted for a game written with clear literary aspirations, by an author whose other work they clearly also like. (Again, I haven’t read Sires yet, so I’m not commenting on how well it fulfilled those aspirations. But have a look at the novels nominated in the last couple of decades; I’ll bet that Sires is more in their spirit than, say, Fernweh.)