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.
The answer to this question can be found in the Epilogue. It begins on p. 414 in the paperback edition of the book. Diamond says that the differences in political unity between these two areas is caused (like most things in this book) by geographical luck. Diamond mentions the following causes:
- Europe's coastline is "highly indented," with five major peninsulas along with two large islands. China's coastline is smooth and its big islands are not nearly as big as those in Europe.
- Europe is split up by major mountain ranges. China's is not.
- China as two major river systems that tie its east and west. Europe has no such things.
For all of these reasons, it was much easier to unify China while Europe remained split into pockets with different languages and ethnicities. This made China more politically unified and kept Europe disunited.
The answer is on pages 409-417. Good luck!