User blog comment:Argorrath/Treyarch posts a message of zombies; possibly hinting to a DLC 4/@comment-5213593-20130804050953/@comment-3967954-20130804082642

I've answered this question many times before, and the answer is so fucking obvious.

Maybe because Treyarch didn't feel like modelling a whole new bunch of weapons for one map? You know they hadn't made Black Ops II's weapons when Moon came out, right?