User blog comment:Deathmanstratos/Black Ops II Apocalypse DLC announced/@comment-4232106-20130808182721/@comment-6343006-20130808183921

Well first of all, the Block Ops zombie maps were on a much larger scale than the World at War ones; trying to recreate them would have likely been too time-consuming for the studio. Secondly, in the Rezurrection case, they already had the updated maps from the Prestige and Hardened editions, so it was a simply process of merely recyling them for the map pack.

So, if they had gone that way, this map pack likely would have cost more time and money to develop, which would have pushed its release date further back and too close to Ghosts, so in a way it does make perfect sense to go this cheap and easy route.