Why did they honor treaties rather than just continuing with open conflict? The juice wasn't worth the squeeze. In general you only fight a war when a) you're being attacked or b) there is something to be gained by doing so that, in expectation, is more valuable than the cost of the war. If there'd had been mountains of gold in Canada to be won, or if the indigenous peoples had put up less of a fight (imposed less of a cost), you can bet the British would not have signed any treaties. Same with New Zealand.