The park was designed as kind of a permanent world's fair... and at a real world's fair, countries usually have their own pavillions. Plus, they wanted the governments to sponsor their pavillions, which didn't really wind up happening.
I think, now, a region would stick out in an odd way among all countries. The UK has a central government, whereas say equatorial Africa does not.
There's a World's Fair in Kazakhstan this year called Expo 2017. Ironically, the setup is quite resemblant of Epcot. Ironically, the theme of the expo is "Future Energy." This is a rendering, I can't find an actual aerial.
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