Google's blog posting didn't give a specific reason for the closure, saying only that the site was a "great experiment which provided us with a lot of material for developing future products to serve our users." The company will stop accepting new questions later this week, but archives of older questions will be available for review.
To me, that sounds like a nice way of saying “it's not making us money”. It also may be a lack of interest and focus on other portions of the Google empire. Either way, within the next few days no new questions will be accepted and sometime soon the site itself will go down.