Plutonium

Because zombies is part of the campaign on BO1 so getting zombies to work required the devs to get campaign to work as well. It's the same exe/game
BO2 has 3 different games/exe: multiplayer, zombies and campaign. So the devs simply worked on zombies in both cases but one game "gifted" campaign and the other didn't. Since Plutonium's goal is multiplayer campaign is not added

To answer OP:

BO3 was one of the first games to heavily introduce Micro-Transactions and ATVI still make money from it to this day.

Plutonium does not want to carry the risk of being legally ordered to shut down by taking these too far and affecting ATVI's bottom line.

There are other smaller issues too but this is the big one, alot of the owners of Plutonium have careers that we can't risk losing just so people can play BO3.