With the original Call of Duty Black Ops, Treyarch wanted to prove that it wasn't the "underdog" of the Call of Duty series. They were set to prove they deliver titles as good, or better, than Infinity Ward. While the success of that game versus Modern Warfare could be disputed, it seems to be widely agreed-upon that it was the best Call of Duty the company had ever created.
With Black Ops II, the developer wanted to up the ante - and by all accounts, they have. Treyarch has proven Black Ops II to be the biggest evolution in the series, with key additions ensuring re-playabilty of the single-player campaign - something pretty non-existent with previous titles.