The sole reason is capacity. Not enough people flying accross the tasman on one airline. NZ and QF have both cut back seats to accommodate this and unfortunately the only solution was smaller aircraft. It has made way for twice daily QF flights AKL - BNE and thus a more flexible schedule between these cities.

The BNE 744 was never going to be permanant and the 767 is now being utilised elsewhere where it can be filled.