Because the Prussians, with their handlebar mustaches and spiked helmets don't look as cool as Nazi storm troopers. And really, what symbolizes the battle between good and evil more than the Allies against the Axis? An alliance of people who wanted to eradicate all undesirable elements from their society based on race, religion, sexual preference, and mental faculties, and who also loved raping and killing Chinese civilians, versus people who wanted to stop the expansion of the former. World War II is about as clear cut as you can get about who was right and who was wrong, and it was an exciting time as far as technological advances. WWI had no genocide, it had no occult obsessed aggressor with a funny mustache, it didn't effect nearly as much of the world, and Warner Brothers never made a cartoon about it. Also, the end effect of WWI and the Treaty of Versailles kind of kicked off the whole German Nationalist movement is a bit of a sore spot. In the end WWI, while extremely important, was nowhere near as important, or as rife with various videogame plotlines as WWII was. And let's face it, there's no group aside from Aliens and Robots that's as globally acceptable to kill in videogams as Nazis.