Under what kinds of conditions does technology develop and how and why did technology development differ from one place to another?
The whole book is really all about this question, so it's hard to boil it down right here. However, I think that it is important to understand that Diamond talks about how technology develops based on how much competition there is between various countries. If, for example, you have a unified empire like China, there is very little technological development.
We also see Diamond talking about how development differs from place to place based on things like how soon agriculture developed in the various places.
These are a couple of major factors, but it is hard to really condense the whole point of the book into an answer of this sort.