The answer to this can be found in the Prologue. There (from page 18 to page 23 in the paperback edition of the book) Diamond lists the four most common answers to Yali’s question. After listing each one, he gives his reasons for rejecting it.
The first answer to Yali’s question is an answer that is based on ideas about racial differences. Essentially, this answer says that white people came to have more “cargo” and to be more powerful simply because white people are superior to nonwhites. Europeans are said to be (on p. 19)
genetically more intelligent than Africans, and especially more so than Aboriginal Australians.
The second and third answers have to do with geography. The second answer holds that people from colder climates (like the Europeans) have to work harder and be more intelligent in order to survive. They have to be more inventive whereas people in tropical areas can just sit around, pick fruit off trees, and not have to worry much about housing or clothing. The third answer says that people in river valleys have advantages over everyone else. This says that people living in river valleys in dry climates needed to have major irrigation works to subsist. This necessitated centralized government to oversee the process. That led to civilization.
Finally, there is the answer that
lists the immediate factors that enabled Europeans to kill or conquer other peoples…
In other words, this explanation holds that Europeans came to dominate because they had things like guns, metal tools, and infectious diseases.
These are, according to Diamond, the four most common answers to Yali’s question.