I think that there is a perfectly good reason for Wakanda's actions visavis the rest of Africa...in fact? Several good reasons.
And I think that RH made a very good argument for almost all of them during his tenure.
Fact is? A lot of us are starting way too late in Wakanda's history and bringing up questions as if the previous history wasn't of the first importance regarding understanding what is happening in Wakanda and why it's happening in Wakanda the way it's happening in Wakanda.
Wakanda has been undefeated for 10,000 years. That literally takes us back to the Upper Paleothic period.
http://en.wikipedia.org/wiki/8th_millennium_BCWakanda has been at the forefront of civilized development in the MU even back then, and was/is unquestionably the pinnacle civilization in every regard, insofar as humans are concerned.
Why is that important? Because it puts Wakanda in the sights of a wide variety of Marvel beings that were interested in Earth for various reasons at that time:
All of the legendary Gods from all the pantheons [ Egyptian/African being paramount, because no other human civilization worthy of the name was in existence in 8000 BC ].
The Kree...who were on a station located in Uranus due to the ramifications of the Kree-Skrull Wars. The Kree were drawn to Earth due to curiosity regarding the actions of The Celestials the genetic potential invested in its populace by The Celestials. They initiated the process...millions of years ago...that would result in The Inhumans. And as time would show? The Kree kept an eye on humanity ever since...
There are The Inhumans themselves, and both their treatment of The Unspoken..the first great king of The Inhumans who Black Bolt in his youthful rashness overthrew...and their creation of The Slave Engine, which The Unspoken decided to never use. Even against the encroaching hordes of humanity; a distinctly like humane but highly unpopular secret decision of The Unspoken's reign which ignited the successful revolt and coup of Black Bolt. [ We know that The Wakandans would be watching this with great interest and suspicion ].
And a lot more...
...so what would stop the crafty Wakandans from playing the long game of building themselves up to defeat each and every one of these extrahuman definite/probable/possible threats, and deciding that humanity's best chance lay in themselves? Not much. And that would be very characteristic of The Wakandans.
The Original Sin of Wakanda might be that she surreptitiously from behind the scenes encouraged the tech growth and warfare skills of humanity only to see it spike to the point that humanity might be able to fight off these extraterrestrial powers...and wreak havoc upon itself. That might also add some serious layers to Wakanda's unwillingness to go further than it already has when the slave trade and other things like The Bubonic Plague came about.