probably because black ops 1 was fun
Black ops 1 wasn't really that good.
It was better than a few other games but it still has a bunch of major flaws.
For example the handicapped weapon balancing in the multiplayer :
You start with basic guns and later you can unlock guns that are exactly like the basic guns just with more firepower / accuracy / whatever.
I gotta give them credit for the extended weapons customization though.
At least the campaign had a decent story but it still had infinite respawning waves of enemies with handicapped AI that just spray bullets in your direction.
Zombie mode was okay in my opinion. Honestly I did not really care about it.
However Treyarch is also just trying to milk money from the huge ass Call of Duty franchise. It wouldn't surprise me if the game turns out to be very medicore and not be as good as everyone thought ( similar to earlier COD games ).
Still , I could be wrong and the game could be better than expected.