The answer to this can be found largely in Chapter 10. There, Diamond tells us that geography put these areas at a disadvantage compared to Eurasia.
In other chapters, we learn that Eurasia had more domesticable plants and animals than Africa or the Americas. This was one issue. But in Chapter 10, we also learn that the orientation of these two areas put them at a disadvantage. Both Africa and the Americas are oriented north to south rather than east to west. This (along with the presence of things like deserts and jungles in both areas) makes it very hard for crops to diffuse across long distances, creating many agricultural civilizations.
Because Africa and the Americas had fewer domesticable plants and animals, and because those that they had could not diffuse as well, they faced a developmental disadvantage.