[World-Building] Hypothetical insular banana republic - why insular?

[Phantasm]

Names removed to protect the innocent - and the guilty.

Let's make a hypothetical insular nation on Lake Victoria in Africa, about the size of Lichtenstein or Andorra, nestled in a sliver of land between Kenya and Uganda. The region has been settled for over a thousand years by tribes descended from both Bantu and Kushan/Nubian stock. The rulers of this area have only recently (say, the last 40 years) discovered a source of Unobtanium present nowhere else in the world, and while they've exported a small amount of the Unobtanium for a sizable amount of wealth, the ruling family has benefited from the wealth the most. This essentially makes the nation a "banana republic", though because they've had a national identity for hundreds of years they're more stable than most. In western political terms, they were part of the British East Africa colony, though they managed to retain practical self-rule even through WWII. This nation declared independence and closed its borders almost entirely in '63 when its neighbor Kenya also obtained independence from Britain.

However, something happened at some point in the past to make them an insular culture, possibly even before the Unobtanium was discovered.

What could this "something" be? Ideas?