Xbox 360 Great game - but why so very short?

[Esperahol]

Outside of the busy work and the achievements the game is actually pretty short. Even with that stuff it only took me maybe 18 hours to get through all of it. That is one of the shortest titles I've played in a dog's age. And I count Arkham City in that score (so very short overall). Maybe it was because people couldn't deal with the short, incredibly repetitive soundtrack you couldn't control?