The Chatham Islands are not New Zealand. They are small, and cold, and you can’t grow the crops you are used to as Polynesians. With limited resources, you could continue to practice tribal warfare and/or one-upmanship, like the Easter Islanders did, and end up with an ecological disaster and mass starvation. Or, you could adjust to your new circumstances.
And the most important adjustment the Moriori made was when their chief Nunuku-whenua, quite early on, decided they should become pacifists. Their situation simply did not allow the kinds of protracted feuds and tribal warfare that they could afford back in New Zealand. Disputes were settled by duels, and the duel was over at the first sign of blood. Nunuku’s Law was that the Moriori shall not kill, shall not wage war, and shall not practice cannibalism.
So, back in New Zealand, the warlike Māori continued to be warlike Māori. When the Europeans came to town, the northernmost warlike Māori got guns. Which they turned on the next tribe south. When the next tribe south got guns, their northern neighbours stopped massacring them; and benefitting from the new balance of terror, the southern tribe turned their new guns on the third tribe south. This continued on all the way down the North Island, until a raiding party went across to the South Island. With guns.
The current Māori population of the South Island is not very large.
About the same time that North Island Māori with guns started killing South Island Māori, some displaced North Island Māori with guns hijacked a whaling ship, and hitched a ride to the Chatham Islands. They then proceeded to eat a seventh of the Moriori, and enslave the rest. They forbade them from intermarrying with each other, and from speaking their own language. Thirty years later, the Moriori were down to a tenth of their pre-invasion numbers.
And here’s the thing: when this was going down, some Moriori begged their elders to let them fight back.
A hui or council of Moriori elders was convened at the settlement called Te Awapatiki. Despite knowing of the Māori predilection for killing and eating the conquered, and despite the admonition by some of the elder chiefs that the principle of Nunuku was not appropriate now, two chiefs — Tapata and Torea — declared that “the law of Nunuku was not a strategy for survival, to be varied as conditions changed; it was a moral imperative.”
The pacifism that Nunuku-whenua bequeathed the Moriori was absolute. And it saw them decimated.