Perhaps the most widely-accepted theory as to why the Maori and Moriori societies developed differently has been popularized by Jared Diamond in his book Guns, Germs, and Steel. Diamond argues that the Moriori, who inhabited the Chatham Islands, were unable to develop the type of intensive agriculture that would support a large population. This was not because of anything in their culture, but basically because of the climate of the Chathams, which was too cool for the crops they brought with them.
The Maori, on the other hand, inhabited New Zealand (indeed, the first Moriori settlers of the Chathams were also from there, and shared the same ancestors as the Maori.) New Zealand had a climate that was much more conducive to agriculture, and so they were able to develop a complex society with superior technology. They also, unlike the remote Morioris, had neighbors who were hostile, so they had to develop a powerful military. When the Maori invaded the Chathams in 1835, the result was predictable. The Maori subjected the Moriori to a bloody defeat.