The main reason for AC operating HNL-SYD return once daily is that the timing seems to make the most sense for business travelers (the flight leaves YVR in the evening, then one can sleep, thus reducing jetlag). At least that's what I've heard. Other airlines do that, too, I think. Besides, I don't think SYD is a 24-hour airport, either. My dad said the schedule hasn't changed by much in 20 years! He knows, as he flies to SYD often.
AC uses the 763 during the northern Hemisphere summer because this is not the peak period for travel to Australia and NZ. Generally, peak travel to Down Under is mainly between October and March, and that's when they use the A340-300 and sometimes the 744.