Yes the J2 attack may be great for Japan……but what about your allies?
In the game Germany and Italy are more effected by the USA than Japan. For the Axis as a whole it seems like the J3 attack is a better idea.
Italy and Germany need more time to secure Africa and Russia before the USA is landing 6 transports a turn in Europe or Africa.
Especially if Germany builds enough infantry to take Russia they may not attack until G3 or even G4. If Japan attacks early it may force Germany to either attack Russia before it is ready, or to wait and then not have enough time to hurt Russia before the USA gets involved with multiple landings.