Well, I think an explanation in real-world terms is needed, not theoretical definitions. For example:

1) Does Xia have better climatic conditions? If so, why wouldn't neighbouring countries have similar conditions to become a viable competitor?

2) Do the have better packing and transportation technology? If so, why can't another country develop/buy/steal the technology?

3) Etc, etc.

Hence, my point is that the imposed constraints may not normally exist.