Why are zombies on black ops?

Answer 1

because the Call of Duty company wanted to copy the idea of Zombies from the game nazi zombies and still put in a one player story mode

Both games are marketed from the same company. Treyarch created the Zombies as a map on Call of Duty World at War and also included an additional map on each of the three map packs. With the release of the Prestige and Hardened edition of the game Call of Duty Black Ops developed by Treyarch those 4 maps were included along with 3 additional maps. Two more zombie maps are available as part of the Map Packs being released for Black Ops

The zombie maps are very popular and that is why they were chosen to be added into the hardened and prestige editions of the game. They are so popular that the sold out games are now selling for $100 more than the original $80 cost