Maybe are working with the chinese authorities tryiing to describe to then the zombies as "healty and young people with a hunger problem" to pass censorship.
Or the physic engine is creating energy from nothing, and a OIL corporations is paying CliffyB so he don't release the game.
Or are stuck on the tribunals, because the game infringe a few patents on Lego Star Wars.
Or maybe the game first player was so good, theres not need to anyone else to play it again and are archived the project.
Maybe have send a alpha build to the England Queen, and she don't like it, and as send a very angry letter to the P.O.U.S.A (president of usa), to stop development.
Or maybe a journalist a raised a "Information embargo" about the game, and the game is released and all, but since all journalist obey the embargo, nobody knows anything about it being released.