We usually refer to these countries as the Axis, as if they were one big alliance during the War. But they didn't behave like proper allies (Japan and Germany/Italy). If they were allies, why didn't Japan attack the USSR from the east? (making them unable to send reinforcements to the west and probably making Germany take Moscow) and why did they instead attack USA, a country that Germany didn't want to see involved in the war?.
To me it looks that Japan did its war without any connection to the European Axis and that this "Axis" denomination dens't really make much sense, rather there were two separate wars (the Pacific and Europe)
For the same reason that the Western Allies didn't open up a European theatre until '43: The manpower and strategic situation never allowed for it.
Japan had, because of its attack on Pearl, gotten itself into an incredibly expensive naval war in the Pacific, and opening a second front as a single nation would have been impossible, given the manpower needed to hold the Home Islands.
Look at the Allies, for an example: The Russians were fighting the Germans since the get-go, and the British, Commonwealth, and French forces were pushed off the continent fairly quickly. Once they got to North Africa, it was a three year fight to break the German forces there and establish a toehold in Italy. Had the Japanese not been involved in a massive naval war, then maybe they could have gone through Manchuko and attacked Russia in the east (moreso than they did, at any rate; keep in mind that there was an undeclared Russo-Japanese war essentially from '32-39, and the Russians invaded Manchuko in '45 and wiped out the Kwangtung army in a few months) though given the setbacks they encountered in the Border War, it seems doubtful they could have done any good to relieve the pressure against Germany.
You should take a look at Alvin Coox's Nomonhan: Japan against Russia, 1939, which gives a pretty solid overview of the whole situation.