Many speculate it's the delay in the availability of the 777-8 that was the deciding factor. We use our airliner performance model to check if this is true or if there are other factors that turned the A350-1000 the favorite.
A350-1000 chosen for project Sunrise. Source: Qantas.
The A350-1000 is a stretch from the A350-900 while the 777-8 is a shrink from the larger 777-9.
A derivative based on a stretch is often more efficient than one based on a shrink.
When Airbus adjusts the Maximum Take-Off Weight and fuel capacity of the A350-1000, it can fly the routes Qantas asks for.