How did the natural environment of the Chatham Islands cause the Moriori to develop in a dramatically different way from the Maori?
They had the same cultural heritage, so why the difference in Guns, Germs, and Steel?
A good answer to this can be found about three pages into Chapter 2. The basic idea is that the Chathams were too small to support agriculture. The Maori remained as agricultural people on New Zealand while the Moriori reverted to being hunter-gatherers.
The Moriori did this because there was no real way to farm on the Chathams. The Chathams were relatively small and cold so they could not support the crops the Maori grew in New Zealand. Because of this, there could be no food surpluses and no large political organizations. Because of this, the Moriori developed into a small group of hunter-gatherers who had little technology and little social organization. This was very different from the Maori who farmed and who organized themselves into what Diamond calls "proto-empires."
On the Chatham Islands, there was no room for agriculture so the Moriori became hunter-gatherers while the Maori were very agricultural.