The answer to this can be found in Chapter 2. There, Diamond tells us that the environment of the Chatham Islands was much less conducive to agriculture than that of New Zealand, where the Maori lived. Therefore, the Moriori on the Chathams came to be much less technologically advanced than the Maori. They also had a less complex society.
Diamond argues that the Chathams were too small and too cold for agriculture. This led the Moriori to become hunter gatherers with a relatively small population and a society that was not complex. By contrast, the Maori on New Zealand had agriculture, which allowed them also to have technology and a complex society. In this way, the natural environment of the Chathams made the Moriori less strong (in military terms) than the Maori, who were therefore able to conquer them.