The thing with WWII is that there was a common goal for the Allies to achieve. Hitler was a genocidal madman leading a conquering country and had to be stopped.
Imagine a game based on WWI. That would be so BORING. The only thing you could do is sit in trenches all day, try to capture enemy trenches by crossing No-Man's-Land, driving ancient "tanks", or piloting biplanes.
No one would buy that.
There are hardly any games based on Korea or Vietnam for a reason. It was simply a time that America wanted to forget.