Because it would cut CoD sales in half. With 80k people playing most of the time (that I averagely see) and 30-40k of them in Zombies. There is no way they will separate that mode from the game, half the population wouldn't bother with CoD anymore.
: ( but I want one so much....
The worst mistake a game developer can make is listening to the community. Halo Reach, Modern Warfare 3 and Black Ops 2 are the ending result.....enough said.
I love zombies but I don't want them to make a seperate game. They would ruin it trying to make a campaign mode. It's perfect the way it is, multiplayer based, no definite ending, endless zombies until death, with backstory sprinkled throughout.
I just can't imagine how they would make a respectable campaign for a seperate game. It would just end up being like L4D, just mowing down zombies from point A to point B. Granted, a game like that with the COD graphics and mechanics would be fun, but I prefer what we already have.
WARNING!!!MAJOR SPOILERS!!!! You are going to die.