Two possible reasons I can think of:

1) SFO slots are subject to control. As SEA_Tigger pointed out, there may not be slots available for this additional service, especially rumors call for SFO-Nagoya services to be launched sometimes in the late fall.

2) Similar reasons as the launching of P.S. services between LAX and JFK - entertainment industry. All the major Hollywood studios are now making movies in Australia to take advantage of lower cost and less restricted union rules in the movie industry. Typically the movies are made during the summer season down under. I have a friend who is in the Art Direction side of movie business, is planning to be in Australia in the winter months (North America) to work on movie projects.