In the past, WWII games were made for quite a few reasons:
- it's a familiar setting. - WWII is very wide in scope, in terms of technology, settings, and plot. - Despite the fact that many gamers got bored of WWII games quickly, most gamers only played one or two of them, and developers really only made one or two WWII games so there were always developers (and gamers) for whom the idea of a WWII game was new. - the biggest-selling FPS games were, in general, set in WWII - Medal of Honor, Call of Duty, Battlefield. For FPS developers thinking about a setting, WWII seemed like a safe choice.
Then all of those reasons became liabilities:
- people became over-familiar with the setting. - The scope narrowed as games started covering everything, from France (including as French Resistance fighter, just to mix things up) to Italy to Russia to Norway to Germany to the Pacific to Egypt and so on, leaving fewer "new" settings within WWII. - Developers got bored of WWII games and gamers got tired of them in significant numbers (a few years after people on games forums on the internet got tired of them). - The biggest-selling games moved on, past WWII, meaning that it looked less like a safe choice. If you are a medium-sized developer wondering whether to set your new FPS game in WWII or not, you'll probably consider the possibility that there's a good reason why EA and Activision aren't using that as a setting.
what about vietnam or something?
Every FPS or TPS game that tried to set itself in Vietnam suffered from bad reviews and/or low sales (usually both). It's possible that it's inherently not a fun setting. Think of the stereotypical Vietnam setting in terms of games - enemies are generally unseen, tunnels, traps....it'd be tiresome. (There's a lot more to the Vietnam War than that, obviously, but those things are what people would be expecting).