I don't know why the developers haven't used much of the lesser known WWII campaigns that have taken place in the European and African continents. However I can understand why they wouldn't use the Second Sino-Japanese War even though it's an atmospheric and emotion gold mine. The problem with Second Sino-Japanese War is that if it's setting is developed into a game now it would stir up quite a bit of a controversy( namely the international political shit storm type). Since that both participant of that war is suffering from some sort of amnesia, one of them cant remember who actually fought for them in the war, the other one is claiming that the war didn't even happen at all.