You can find the answer to this question in Chapter 3, pages 78-9 in the paperback version of the book. There, Diamond says that literacy and well-organized government helped allow the Spanish to conquer the Inca.
The reason for this was that an organized government could easily command the resources of a country. The Spanish could use all of their riches to create the military force that conquered the Incas while the Incas could not remain organized after Atahuallpa's death. This was exacerbated by the Spaniards' ability to read and write. This allowed their governments to accurately gather information and transmit it to those who needed it. If the Incas had had this, they might have known more about the Spanish and been better prepared to deal with them.