This may still be because of ETOPs for technical reasons. Yes, the SYD-JNB route is approved for ETOPs 330, but it also means the diversion airports need to be operational at the time of the flight and can accept the size of plane in question. Given that airfields in the Southern Ocean are few and far between, and likely very much affected by weather, the number of days in a year where the route can be operated without a major flight deviation on a twin engined plane.

The A380 offers year round stability on the route.