User blog comment:Damac1214/Sales of Black Ops II are a "Cause of Concern" for Activision/@comment-402843-20121201183222/@comment-1187559-20121201183643

Coding new engines is hard. Which is why most popular, 'continued output' games use modified engines. If you look at the gap between TES IV and TES V that was mostly from having to build the new engine.