I don't think he has enough troops in eastern asia yet to do this. Plus, if he goes straight for the mainland, he runs the risk of getting cut off by Japanese Naval forces (as nearly happened in his recapture of Macau) If he can sieze the ports of the Japanese that are near his axis of supply running through the suez, and then indonesia he stands a better chance of getting troops and supplies into the theater without having to devote massive naval forces for escorting each group of transports.
Maybe your right, I have never played the game with such an extended supply route. Indochina looks ripe for the plucking, if he doesnt take this region this will always be at his back (plus Axis Siam).