I am not sure, we can only speculate. But when Fastjet introduced the DAR-NBO route and dropped the prices, KQ followed. I guess TC might not want to take on KQ in an open price war right now when there are other routes currently not served at all. And as you say yourself, I dont think there is that much traffic on the NBO route now compared to the past. Also, I have a feeling that a lot of the passengers flying DAR-NBO on KQ are connecting on to destinations such as BOM, CAN and JNB, so they would not choose TC.
BOM and CAN are in the TC pipeline, should happen mid this year. I would be surprised if JNB didnt happen also sooner or later.