You can find the answer to this question in the Prologue. There, Diamond tells us that Yali's question interests him because of the things that he experienced as an evolutionary biologist. Diamond was interested in evolution, of course. And he became interested in human societies as he saw many kinds of technologically primitive societies while he was studying things like bird evolution. In addition, Diamond lived in Europe for a while and saw how much people there had been affected by WWI and WWII. All of these things led him to think about broad trends in human history and why those trends come out as they do. This is very much in line with Yali's question and helped cause Diamond to be very interested in the answer to that question.