The two reasons that I’ve heard why the B777 will get the Sunrise gig:
The cost benefits of the B787/B777 single endorsement are more significant (& longer) than the A330/A350 cross crewing. The A330 will be retired by 2025 and there is quite a technology difference between the two Airbuses.
Also, the current QF management have a bit of an anti Rolls-Royce thing going and that’s the only engine available on the A350.
The fact that the A321’s have been already ordered indicates that the B737 Max won’t be chosen, irrespective of how good a deal Boeing offers. The Max is also very poisoned in the minds of the travelling public.