There's not many other places to go besides the future, especially if you want the developers to be "creative" rather than a lot of it being copy/pasted from history. That was something specifically addressed by Sledgehammer in Game Informer. They could've gone back to a WWII setting or some other past events, but they would be restricted on how creative they could be because they would be shackled by having to remain largely historically accurate (and I'm sure many people would complain if they made it into some alternate universe setting because that would also be "too different").
And even if people never said outright that they wanted a "futuristic game", they technically got one with CoD4, MW2, and MW3 anyway (2011-2016, with them coming out in 2007, 2009, and 2011). As I mentioned though, the reason that doesn't seem "futuristic" is because it uses a lot of stuff that is very easily recognizable and don't take into account things in planning stages or prototypes, and other things much. BO2, Ghosts, and now AW are taking those kinds of things more into account while making them more "realistic" by moving time forward to a more reasonable time where such things could potentially be much more commonplace.