There are two different ways to answer this question.
First, you can say that Diamond gave no answer. This was the case when Yali first asked Diamond this question in 1972. Diamond did not know how to answer Yali at that point.
Second, you can say (as Diamond does) that this whole book is Diamond's answer to Yali's question. In this book, Diamond essentially says that white people got so much "cargo" because they were lucky. White people, originating in Europe, had geographic luck that allowed them to develop farming, civilization, and technology sooner and to a greater extent than anyone else. Diamond emphasizes that this had nothing to do with the racial or cultural qualities of white people or of Yali's people and others who have been dominated by Europeans. In this way, then, Diamond's answer to Yali is that the geographic luck caused the disparities that Yali observed.