I recently read The Defences of Macau by Richard J. Garrett which touched on the conditions of Macau during the second world war but fell short of explaining why Japan didn't invade as they did with Hong Kong. Does anyone here have any insight from the Japanese perspective?
Oddly enough, the answer to this one is fairly clean-cut: Portugal, the owners of Macau at the time, were neutral in the war, and Japan decided to respect that neutrality.
They did station Japanese "advisors" in Macau, however, to prevent it from being used as a way to bring arms to the Chinese. The Japanese also were hardly shy about throwing their military weight around, such as kicking the Portugese garrison out of nearby Lappa Island.
So there really are two answers: 1) The Portugese were neutral in WWII, and 2) Japan really didn't need to, they had de facto control over Macau anyway.
Good reading here: The Macanese at War
Note: fun question!