That's not exactly true, according to WGA spokesperson Micah Wright, who wrote a guest column on the topic for GamesIndustry.biz. Nominees, as it turns out, actually only have to be a member of the Videogame Writers Caucus (for the princely yearly sum of $60) rather than the full union.
So what's been keeping some of the best games of the past four years from being nominated? Some developers -- like BioWare -- simply haven't submitted scripts, but with others it's a question of not knowing who to nominate. "The first year, several games which people believed should have won our award were not eligible because the Developer didn't bother to credit the people who wrote those games," Wright wrote. "That's an insult, and as a guild of professional writers, we're not about to give an award to either a mystery person, or to a company which can't be bothered to honor the workers who made their game."
Wright also quipped, "We're not giving an award for 'Best Videogame of the Year!' - if you want that, go watch the Spike Game Awards." We hope the WGA has the wisdom to nominate that for "Unintentionally Funniest Line of the Year."