Likewise BA used to have one daytime flight from CPT (seasonal iirc).
And SA's craft spend the day idly at LHR in similar fashion. I would imagine that one reason night flights are popular is for connections. A daytime flight of 11 or 12 hours means that arrival is too late for most connections.
I have heard that BA have their craft cleaned in JNB to utilize some of that time and because it's cheaper than at LHR but no idea if that is (still) true.