I think the routes had several problems all at once.
First, the economy tanked. If people aren't buying the J seats, I doubt the route is sustainable.
Second, NBO got caught with a tanking economy AND violence in NBO. I'm sure DL didn't want to fly into that (pun intended) particularly.
Third, security is an issue in African airports.
Fourth, Dl may end up petitioning to do some code shares to NBO and other cities using associate member Kenya Airways.
Fifth, Cape Town has been experiencing significant crime, or at least was last year when I was there. Not only are many leaving the area, the police publicly suggest that homes not only have private security, but armed response should be added (by the alarm company). I think longer term CPT will be a good destination for DL. But right now, perhaps, there isn't enough support for this and a variety of other reasons. J'berg is probably a better destination right now for biz reasons.
Dave