You kids don't know nothin'. Back when I was in Nam, it was a different story.....
jk, jk. xD
Nah, I think the reason is that FPS games are like a cycle. The first half of this decade there were a few hundred WWI/WWII games released and everyone was playing one.
Then CoD4 came out and you saw an influx of more modern shooters, kind of in the Afghanistan/Iraq era. A lot of desert maps and a lot of scenarios with Islamic terrorists.
Now we have Black Ops, set in the 1960s, which is starting the new trend. No more predator drones, no more UAVs... nope, we're goin' back to Nam.