Originally Posted by
kayexalate
Is it because it is 2 different airline partners and neither of them are AS? (the LAX-SYD is always Qantas).
Yes. You can only assemble mixed carrier itineraries with AS +
one partner airline at a time, but not two (with the exception of AF/KLM that count as one). You might have better luck finding just the Australia flight from LAX, and then buying a cheap cash ticket SFO/OAK-LAX.
Also, don't miss that Cathay Pacific flights can't be searched for on the website - SFO-HKG-SYD is quite the detour, but it's a possibility, and the extra work needed to book the tickets occasionally means that there's less competition for the award seats.