SIN-MEL and vice versa is a prime last minute full fare 'J on the day' route. As such any prime revenue generating set is saved for this purpose. Holidays are the only time it gets 'opened up', but is usually full in any case. There used to be load/performance issues out of SIN, but these days it is mostly rev potential at the higher end that gets the Rev Man types salivating.

EDIT: last minute cargo is also a huge revenue source on the route, and weight is also saved. It is not uncommon to find last minute changes to load and fuel as a result.