Perhaps because they are very very different airlines. QF large install base of pax with lots of FF points (credit cards, kettles, insurance policies - just take a look at the annual report to work out that the profit centre of FF almost outpaces the international division!) all wanting to get OUT of AU.
SQ, MEL/SYD/BNE flights are all essentially regional feeder flights that then connect with a vast network from a hub (i.e. not just LHR but all over EU, Asia, US etc).

Maybe also QF pissing off QR so that QR restricts FF availability to QF FF thereby limiting supply even more, probably also distorts the market...

(You could probably also argue into this, that QF is much much more profitable - for the airline - as a FF system, they have Aussies right royally stitched up)