Because QF are horribly useless on this route (and many others I won't get into).
I just booked a AKL-LAX-ORD-LAX-AKL on QF and AA and both flights between NZ and USA are on the 743
I'm going to get that flight cancelled and switch to NZ and UA instead because I'm getting pretty sick of trying to book QF25 and QF26 with the dates I want to travel with either a 744 or an A330 rather than the crappy 743.