Because after QF9 departs, (last departure of the night), the evening staff go home. Not much point coming back to Melbourne when there is no staff there to see the aircraft back in. The airport maybe "24 hours" but Qantas staffing levels are not. The earliest the aircraft could be received back would be 0430 at which point it would have been better to go to Sydney anyway, as that is where a spare aircraft and reserve crew would be most accessible.
At the risk of being accused of being an Airbus hater (again)......friggen POS A380
ahhhh well, at least they are consistent.
TOR