Why do some great games sell terrible?

Games like Enslaved and Majin were quietly passed over by most gamers this last fall. Was it because bigger, and more popular games came out at the same time, say like Call of Duty Black Ops, or Fallout New Vegas? Was it the lack of marketing? Consider this, it could have been all of those factors, and one way to fix these issues, is to release what is known as DLC Prologue, and a shining example is Dead Rising 2's Case Zero.