I think you see a lot of WW2 because it was pretty much the first war I'm aware of that had all the hallmarks of a modern war (tanks, aircraft, automatic weapons, etc); in addition to that there was a relatively defined 'good vs evil' element with regards to the Nazis. It's kind of a classic war, if that doesn't sound a bit cold. :~
I assume wars such as Vietnam and the Gulf War aren't covered so much because the USA didn't really accomplish a great deal with those, and the current Middle East war is a bit too current and sensitive to go into too much. Also the nature of it (massive civilian casualties, suicide bombings, etc) is a bit dicey to make a game out of.
I am noticing we're seeing a lot more fictional modern war games now though, such as imaginary current-day wars against Russia and China. And there's no end of futuristic war games now, although it is getting a bit old with space marines fighting against aliens in every other game.
is pretty cool though. It's coming out this month, and is kind of a fusion of futuristic war and WW2: it's set in the future, but aesthetically and themetically it's got quite a strong WW2 vibe about it (just look at the picture on the right). Very excited about this — especially considering how bloody fantastic it looks