African diamonds are located in very small specific locations. One could certainly capture and hold those spots and extract the diamonds profitably. What one cannot do is conquer and rule vast areas of countryside profitably.
The only guy I’m aware of who ever did it was Leopold of Belgium in the Congo Free State, and he did it by methods the Nazis might have flinched at.
Conquest and rule of a vast, poor country is just not profitable, unless there is some resource there in great demand elsewhere.
Which is why the ‘colonies’ work out better.