You can answer this by reading in the Epilogue. There, Diamond tells us that geography is responsible for the differences in how much China and Europe were able to become unified politically.
Diamond says that geography makes Europe less able to unify. Europe's coastline is very irregular, with lots of indentations like the Iberian Peninsula and Italy that could become their own countries. Europe has mountain ranges that carve it up. The regions of Europe, then, were very separated from one another and developed distinct languages and ethnic groups. China has none of these geographical divisions. Therefore, it has historically been much easier for China to remain united and for Europe to be disunited.