Simple business economics. Qantas wants to optimise profits. Aircraft usage is not profit. Especially wide body with very expensive crews.
This.
And they don't even fill the JFK tag, even with three inbound flights meeting it.........so flying anywhere beyond that doesn't make sense. They tried with Vancouver and it failed.
And LAX isn't like SYD - they aren't overcharged for parking aircraft and there is more downtime to actually do some maintainence, rather than in SYD.