The reason for this has to do with climate. Think about what happens to the climate as you move in a line from north to south. Typically, the climate gets warmer as you go (if you are in the Northern Hemisphere). Now think about what happens to the climate as you go from east to west. In most cases, the climate does not change all that much.
Now think about what that means for plants. Plants that can thrive in one climate can typically not thrive in another climate. That is why people in Florida can grow oranges but people in New York or Indiana cannot. When you move along an east-west axis, the same plants can grow all along that axis. That is why corn can grow in Washington State and grow in Minnesota, more than a thousand miles away.
Thus, a crop can diffuse along an east-west axis. A crop that is domesticated in one area can be borrowed by people living to the east or the west of that area. This allows continents with long east-west axes to have crops diffuse, helping civilization to emerge all across the continent. This is not so possible in land masses with north-south axes.