SQ have an A330 on the route, Scoot have a B787 and J* an A320 so the high and low ends of the market are well catered to already with direct flights. With a flight time of around five hours it's not really worth the inconvenience of a plane change to save a few dollars.
Perhaps QF are trying to cater for the passengers who still want an Asian stop over on the way as they can connect with EK and other code share partners Singapore.