Mainly I suppose because of the lack of regional network connectivity.
The problem is not a new one either. Voice traffic had to for many years be routed through Europe and other international points simply because of the scarcity of cross-border routes. Submarine cable networks now have numerous landing points on both the East and the West coast of the continent. Cross border routes are now available, to most SADC countries.
BTW, the REN networks in Africa have been benefitted from this idea for almost 8 years now, with SANReN hosting the interconnectivity. The exercise was precisely to demonstrate the advantages of keeping continental and regional traffic on-continent.