The answer to this can be found in the Prologue. Beginning on page 17 in the paperback edition of the book, Diamond gives three reasons why some people might say that we should not even try to answer Yali’s question.
The first of objections is that answering the question will justify the fact that Europeans have dominated the world. If we explain why the Europeans became dominant, we will essentially be saying that it is right that they got to be this way. We will be saying that we cannot change that situation and should not try.
The second objection says that answering Yali’s question will
...automatically involve a Eurocentric approach to history, a glorification of western Europeans…
In other words, answering the question will simply lead us to think that Europeans are more important and better than everyone else.
Finally, some people object that answering this question will make it sound like civilization is good and “primitive” societies are bad. It will make us prejudiced against people who live as hunter-gatherers because we will think that our own societies are more advanced and are better than those societies.
Obviously, Diamond rejects all of these objections and goes on to answer Yali’s question in the rest of the book.