The answer to this can be found in Chapter 10. There, Diamond tells us that a land mass with a long east-west axis has advantages in the spread of agriculture and the complex societies that agriculture allows.
If one society domesticates a crop, it can spread to neighboring societies and those neighbors can then become agricultural without having to domesticate the crop themselves. This process of spreading (diffusion) is easier along an east-west axis. This is because areas on the same latitude will generally have similar climates and be able to support similar plants. By contrast, if a land mass has a long north-south axis (like the Americas or Africa) crops cannot diffuse as easily and as far because climates tend to change as you go from north to south.
Thus, land masses with long east-west axes develop agricultural societies more easily and become powerful more easily.