Why is this not the real story?
i have no insight and haven't looked this up. But given the SYD flight are among UAs longest, the potential for this kind of situation with a diversion are quite high. Don't know the details of crew regulations or what UA allows in their contract, but if the diversion to AKL, combined with the new time requiired to get to the destination is beyond the legal working hours for the crew, the crew times out, and they can not leave until they have received minimum rest, or UA can get a new crew there. Clearly, this was the option that UA chose given the alternatives. UA doesn't have extra crew in SYD/AKL. The only real otheroption to consider may have been to stop in HNL, and gather a crew while enroute there to replace the ones onboard and continue on. This has other consequences down the line (another stop for pax, two crews out of position vs. 1, etc.) and may have not even have been an option. UA would have evaluated alternatives before deciding. None is going to be optimal, as even if they could have continued on, they still have a crew and aricraft with additional schedule after arrival and UA may have to scramble to make alternatives for.
Crew time outs can especially be an issue on these long flights, where there isn't much of a buffer available. SYD/AKL/SIN and EWR-HKG/India are some of the more likely flights to run into this scenario - a 1 -2 hour delay is enough for the crew to time out.