In chapter 16, I'm not sure if the answer to this is "It didn’t have the geographical obstacles to unification. It was geographically linked and that enabled it to be politically linked. I also don't know if it is saying that China's North/South axis an issue.
China's North/South axis was not an issue because it did not prohibit agriculture, technology, or communication.
While China's north-south gradient retarded crop diffusion, the gradient was less of a barrier there than in the Americas or Africa, because China's north-south distances were smaller; and because China's is transected neither by desert, as is Africa and northern Mexico, nor by a narrow isthmus as is Central America.
China's long east-west rivers (the Yellow River in the north, the Yangtze River in the south) facilitated diffusion of crops and technology between the coast and inland, while its broad east-west expanse and relatively gentle terrain, which eventually permitted those two river systems to be joined by canals, facilitated north- south exchanges. All these geographic factors contributed to the early cultural and political unification of China, whereas western Europe, with a similar area but a more rugged terrain and no such unifying rivers, has resisted cultural and political unification to this day.
Guns, Germs, and Steel
