That has been speculated for pretty much every COD. I think for the historical environments, WW2 is prefered from a marketing point of view as its a more feel-good war. The nazi's were very much the bad guys, were the clear aggressors and invaders and it ended well for america, it won and became the first super power complete with nuclear weapons.
In terms of gameplay, though, what difference does that make? Not much... the environment is irrelevant - as long as the time period supports all the classes of firearms. It could be ww2 or science fiction or anything in between, you're just changing models and skins for the same engine... essentially the environment is dictated by careful marketing rather than an artist choice or anything to do with gameplay. I'm sure its a risk on its own though to keep presenting the audience with similar eras. of course there's also very little difference between far eastern jungle in 1944 and 1968... you can see plenty of recycled objects between even the ww2 and modern war environments.
Basically, while the big but simple games are still selling bucket loads, why try harder? also the total dominence of certain franchises mean that other shooters wont sell, so why put a lot of effort into innovation and making something thats better, why take the risk if you're not sure it'll sell.
David Braben - developer of elite, among other things has written some thoughts about the gulf between what we should expect and what we actually get from successive generations of hardware and games. His conclusion is that nothing is really next (current) gen other than in terms of making a pretty screenshot. His current development of 'the outsider' is something that apparently will feel as next gen as it looks... mindless hype? i hope not as this will no doubt effect Elite IV, which i know many of us here are patiently waiting for.
Its also interesting to see how mobile, small scale platforms have really taken off, meaning many small independent studios can compete where traditionally they'd have little chance against the big studios on large projects. Not sure if we're seeing a huge amount of innovation there, or are just revisiting favourites from yesteryear in one form or another, the 2d platformer, the 2d shooter and so on that many of us grew up with.