What primary environmental factor allowed wētā to evolve to such sizes in New Zealand?

Answer

The absence of mammalian predators.

The text states that the evolution of such significant gigantism occurred without the pressure of mammalian predation that kept insect sizes in check elsewhere, due to their isolation on the landmass of Aotearoa.

What primary environmental factor allowed wētā to evolve to such sizes in New Zealand?
animalsizeinsectrecordWeta