One thing that's become increasingly apparent in today's age of digital downloads - they can disappear at anytime. Most times this happens because of expiring contracts between content providers. Such is the case with DOOM on Xbox Live Arcade. The game was originally released before Zenimax Media actually owned id Software.Once that contract expired there was no place for your money to go when purchased, so the game has been pulled.
The good news is Zenimax will be getting the title back up shortly under a new contract. So if for some reason you don't own the four player co-op shooter yet, you'll still get your chance.