To understand this, think about climate. Climate stays more or less the same as you go from east to west. Of course, there are differences near oceans and there are differences if the elevation changes. However, the climate stays more or less the same at the same latitude. By contrast, climate changes radically as you move north or south. The climate in Seattle is very different from that in Los Angeles.
Because of this, it was much easier for agriculture to spread on an East-West axis. A crop that was domesticated in one place could be spread to other cultures at the same latitude. This crop might not be able to spread north or south because of differences in climate.
Diamond believes that this is a major reason why Eurasia (long East-West axis) produced so many civilizations while the Americas and Africa (long North-South axis) did not.