The answer to this question can be found in Chapter Two of this excellent historical study of global historical development. The author makes it clear that the way in which the Maori were able to so easily enslave and kill the Moriori was a result of the disparate development of these two island societies. It is all down to environment, as the chapter makes clear. The Moriori were forced to become hunter-gatherers because of the different climate of the Chatham Islands that made growing their crops impossible. Because of this, they did not produce enough food to enable nonhunting craft specialists. Their prey could be killed using nothing more technologically sophisticated than clubs. The distance of the Chatham Islands from any other islands meant that the Moriori had to remain on them and learn to get along with each other peacefully, renouncing war, resulting in a small, technologicaly unsophisticated civilisation without organisation or leadership.
In contrast, the Maori developed on the warmer part of New Zealand, where they were able to engage in farming, increasing in population and thus developing specialists who could advance their level of technology. Thus we can see how one common group of ancestors developed very differently because of environment:
The outcome clearly illustrates how environments can affect economy, technology, political organisation, and fighting skills within a short time.
The stage was set for the conflict that led to the colonisation of the Chatham Islands.