Umm… huh?!
I am 36 and have been into gaming in various forms (arcade, PC, console, tabletop, pencil-and-paper RPGs, dice, cards… you name it!) for about 30 of them. I am rather neutral towards the WWII genre and a younger (age 21) gamer-friend of mine is totally sick of it due to market saturation, so it’s safe to say that the appeal is not universal.
However, for a video game, WWII is the one war that people can relate to and not risk causing too much outcry. If we tried an earlier war. the tech would not be there so the market appeal would be more limited since many gamers like fully-automatic weapons, air support, grenades, and other things that were not really widespread until WWII. If we go later, well, we didn’t exactly win Korea or Vietnam so you would lose gamers that like being on the winning side (i.e. most of them) and anything dealing with Iraq or Afghanistan strikes too close to reality, thus causing mixed feelings, possible controversy, and questionable marketability.
Now, we could do the Metal Gear Solid thing and make up our own “secret” history that is similar enough to ours to feel real while avoiding some of the issues that an Operation:Enduring Freedom FPS would face, but the “realism” is limited.
Or we could just leave the whole war/FPS genre entirely. Look at the top 50 best-selling games of the decade has more Mario Kart, Pokemonm, Grand Theft Auto, and Gran Turismo games and only one WWII game on there.