User blog comment:Callofduty4/Black Ops III Zombie reveal at SDCC/@comment-1263839-20150625205941/@comment-24756040-20150626030130

Raven, not everybody plays all of the modes in Call of Duty. There's quite a few people that play Call of Duty purely for Zombies/Survival/Extinction. Same goes for Multiplayer.

There really isn't a primary game mode, as the game is split into three (most of the time), and none of them are really a "primary" game mode. Popularity alone doesn't decide which game mode is the "prime" game mode of a Call of Duty game, or practically any video game split up into multiple core game modes.