Why did Jared Diamond write the book Guns,Germs,and Steel?Why write about this subject? What was his intent and how did he go about writing it?
One of the things that becomes apparent from reading any of Jared Diamond's books is that he is incredibly fascinated by the ways that societies have developed. So I would argue that a major part of the reason he wrote the book is that he has been studying societal development for most of his life and it is his great passion.
Another reason is of course that he felt it was compelling and that people would like to read it and it never hurts, financially or otherwise, to have an NYT bestseller with your name after it. It would certainly make future studies of his easier to get funded, etc.
Another is a desire to tell what he's learned. Most of us, if we've picked something up, like to share it. And Jared Diamond is no different. He feels he has hashed out some pretty interesting stuff about the way societies developed so he wanted to share.