The availability is low. There are not enough copies to meet demand. Right now GameFly is has way too many members for the number of games they have. They won't directly admit that, but it is the case. This answers board is full of unhappy customers. It stinks, and I understand your frustration.
One thing you could do to try and get the game is empty your GameQ of all but that game. Sometimes that works, other times you just end waiting for no game to show up at all.