I think why WW2 games are done to death are it was a time when the baddies were known and in reflection are still baddies. And as well it's a point of pride with Americans, it was the last "glorious" war. Korea, Vietnam, the Gulf War, Iraq, and Afghanistan are all sheep herders with rifles (probably the same ones used by U.S. troops in WW2) versus, Nato/America with ICBMs and billion dollar bombing jets. Killing a person who has no real defense is like punching a baby in the face - a dick move. And no one likes to feel like a dick in their games especially when most of the time games are built to prop up the heroes.
The one that I'm sad was canceled was six days in Falujah, one that would show a real to life approximation of the war on the ground. And everyone protested saying it's insensitive. I personally think it would be a great thing to show what these soldiers go through, but I guess it would also show the senselessness of the war etc. etc.
Bottom line,
WW2 games sell, because killing Nazis is satisfying and it makes us feel patriotic.