Valve apparently informed him at GDC that it would disappear, which is something Gabe Newell has already revealed would happen eventually. It's because it 'won't be needed'.
Greenlight was introduced to give the Steam community the chance to vote for what titles they'd like to see come to Steam, but the decision ultimately rested with Valve.
"We met with Valve at GDC, and they say Greenlight will be gone within 12 months. They'll still offer curated space, but otherwise be open," Jonathan Biddle. Exactly how Valve plan on phasing out Greenlight as unnecessary remains a secret, but they intend for Steam to become 'more open' with less barriers to indies.
Valve recently empowered developers to manage their own sales promotions through Steam.