In this book, Diamond’s major contention is that the people who developed agriculture first became the powerful people in the world. When people developed agriculture, they were able to have civilizations. When they were able to have civilizations, they developed technology and they came to have infectious, epidemic diseases. It was the technology and the diseases that ended up helping them to conquer the rest of the world. Diamond says that the people who lived on east-west axes had an easier time developing agriculture than those who did not.
If there is a long east-west axis on a land mass, it is easier for crops to diffuse along that axis. A crop that is domesticated in one place can move along that axis. This is because, all other things being equal, places along an east-west axis have relatively similar climates. This is not true as one moves north to south. If people lived on an east-west axis, they could borrow crops from other areas. This meant that agriculture could spread more rapidly along an east-west axis. More places with agriculture meant more places with civilization. That meant more places with people who could become powerful.
This is why people who lived on east-west axes came to dominate the world.