Why wasn’t T'Challa played by an actual African, since the fictional character resided in a fictional country within the continent of Africa?

Well for the same reason Benedict Cumberbatch (a British man) played Doctor Strange (an American man)…

For the same reason Tom Holland (a British man) played Spider-Man (an American man)…

And also for the same reason Bradley Cooper (an American man) played Rocket Raccoon (a space Raccoon)…

This list could go on….

The point being that it is down to the individual actors skill of being able to convincingly pull off the character!

Chadwick Boseman did a brilliant job of playing Black Panther in my opinion as it is down to his skill as an actor.