The answer to this can be found in Chapter 12. There, Diamond tells us that some people did not develop writing because they did not need it.
Writing, Diamond says, is not something that you would invent just because. There needs to be a good reason to go to all the trouble. In general, this reason only arose with agriculture. When societies developed agriculture, they came to be more complex and wealthier. This led to a need for writing. Writing was necessary to keep track of inventories and for governments to keep track of who owed how much in taxes.
Thus, the people who developed agriculture were more likely to develop writing. Societies that did not develop agriculture or who developed it late were less likely to develop writing.