Probably because the holocaust and bombing of Pearl Harbor make the Nazi's and Emperial Japan such great villians. Think about how few games are made about the Korean War, Vietnam war, or War of 1812.
People love to make games about WW2, American Civil War and the Revolutionary war where they have a wonderful enemy to hate on.
Also a number of aerial combat games have been made about WW1, which if I recall correctly involve killing the evil red baron....







