ZombiU because the team was able to accomplish all of its goals with the game itself, multiplayer director Bogdan Bridinel says. Nintendo Life quotes Bridinel from an interview with Official Nintendo Magazine, where he describes ZombiU's development and editing process.
"A few things that we tried during production ended up on the cutting room floor, but this was because they didn't work out as well as we had hoped, not because the schedule forced us to," Bridinel says.
If ZombiU turns out to be extremely successful, Bridinel does think DLC "could become a possibility." So really, if ZombiU never gets any DLC it's all your fault. Just keep that in mind, zombie killers.