CODM zombies was nothing compared to its PC and console counterparts. People were tired of playing only on one map, and that made it more boring. There were not many guns in that mode, even the mystery boxes don't give you a quality weapon. Zombies were just too to kill.
2024's Call Of Duty: Modern Warfare II featured 2-player co-op missions and 3-player Raids, for instance. In Call Of Duty: Ghosts, players faced off against alien monsters in that game's co-op Extinction mode. But Sledgehammer Games is behind this year's upcoming title and it appears Zombies will make a return.