According to Diamond, there are two different parts of “the East” that lost their lead for two different reasons. These two parts were the Fertile Crescent and China.
The Fertile Crescent lost its lead for environmental reasons. It used to be a very fertile place. However, it did not have the characteristics needed to support large populations for long periods of time. Big populations destroyed the environment and the area was no longer able to be the center of a powerful empire.
China, by contrast, lost its lead because of its political unity and dominance in its region. China did not have any rivals in its region of the world. This meant that it had no competition. Because it did not have to compete with any rivals, it did not have to keep progressing technologically. By contrast, European countries did have many rivals. Therefore, they did have to keep progressing. This meant that the European countries got stronger while China stayed stagnant.
For these reasons, both of the areas of “the East” lost their lead to the West.