You can answer this question by looking in the Prologue. There (pp. 17-8 in the paperback edition) Diamond lists three reasons why some people would object to answering Yali's question. These reasons are:
- Answering the question would justify the fact that Europeans have dominated non-Europeans. In other words, it would make it seem as if European dominance was a good and a natural thing.
- It would "automatically involve" Eurocentric history. This approach to history would glorify Europeans and focus on them to the exclusion of other people who are just as important.
- It would imply that "civilization" is a good thing and that hunter-gatherers have terrible lives. In saying this, it would be biased in favor of modern societies which clearly have their own problems.
Diamond, of course, rejects these objections and chooses to answer Yali's question in the rest of the book.