Diamond wanted people to think about history differently. He also wanted people to realize that everything is interconnected. We are used to thinking about history in a certain way, based on our old textbooks. There is so much more to it.
In many ways, Diamond answers this question himself in the introduction, "Yali's Question." He is arguing against older interpretations of history in an effort to overturn racial and cultural explanations for why nations developed the way they have. I think (and his footnotes back me up here) that he also wants to argue for the importance of a multidisciplinary approach to doing this kind of scholarship.
It seems to me that Diamond is very against theories of world history that depend on the idea that whites are either genetically or culturally superior to other people. Diamond saw a lot of "primitive" cultures while doing his field studies as a biologist. I would guess that he came to feel that they were just as good as anyone else and that there had to be some better way to explain why they were "backward."