In the Epilogue, Diamond speculates that this result was caused by geography just like most of the big trends in world history. Diamond says that China's geography allowed it to have a single, unified government. By contrast, Western Europe was divided up by mountains and coastlines, making it possible for many small countries to arise. The countries of Western Europe had to compete with one another. This made them strong and forced them to adopt any technology that they were exposed to. By contrast, China was a unified empire with no rivals. It could do things like abandoning ocean-going ships without fear of being dominated and conquered by its neighbors.
Because China did not have to compete, it lost its edge and did not become a global power.