The series started at WW2 (Original CoD), then did it 2 more times (CoD 2 and 3), then went back for WaW.
Unfortunately, modern/futuristic shooters are the norm and what's popular right now. I personally think this will be the case for a few more years. I believe this because most of the modern shooter titles started with WW2 or covered it at some time (Medal of Honor, Battlefield, CoD), and are now in the modern/futuristic niche. I believe at some point this will get dull, as did the WW2 theme for most, and we will see a return to historical shooters.
WaW remains my favorite CoD for many reasons, probably because I am a WW2 enthusiast, but I am pretty happy with the direction that shooters have gone.