I believe we got the countries we got because either their tourism boards or private industry were willing to fork over the funds to help get their pavilions built. I don't know if it was when it started but when it ended, Norway, was supported by a private sponsor, for instance.
I'm sure that late 70's to mid 80's international politics had a lot to do with who had their check books out and who's calls Disney was taking on the matter, too.
It was my understanding, for instance, that Israel wanted a spot for years but was turned down for the obvious reasons.
I remember they
did manage to get into the World Showplace pavilion during the Millennium Celebration and they were the only country there that had an actual ride. (it was a simulator)
There were plans announced when the park opened for an upcoming Equatorial Africa - none other than Darth Vader himself* announced so on the park opening TV special. Personally, I'd very much welcome that addition if someone somewhere would pay to have it added.
*of course, I mean this guy and if I remember correctly, he was standing next to a Fire Starter-era Drew Barrymore when he said it.