Part of CX's business strategy is to create a strong network with HKG as its hub, providing lots of connections for pax travelling through HKG. To that end, they prefer providing a higher frequency of service with a smaller aircraft, rather than one flight a day with something like the A380.
As ACMS said, CX worked out long ago that there aren't many routes where they could make money with the A380 in its current form, severely limiting its flexibility. I haven't operated the South Pacific, but winds of 200 knots aren't uncommon over the North Pacific in winter. Westbound flights from North America to HKG are usually payload limited and very tight on fuel, resulting in tech stops more often than not.