Essentially too many travellers and not enough aircraft. CPT is very popular with English speaking travellers, Same timezone, same language, much better weather and depending on the Rand's fortunes it can be good value for money in Sterling terms. But the route options are quite limited, being stuck at the wrong end of Africa in terms of aviation flows. Competition is somewhat throttled and though EK have some impact it is a huge amount of extra flying. There's also a large local UK centred population in the Cape who won't touch SAA with a very long barge pole (though they aren't really terrible). So they work / study in the UK but their family is in the Cape and they travel home twice a year. Plus all the UK/SA business travel. And so it's not actually in BA's interest to greatly expand the seating numbers to SA, apart from WT, seating availability can be quite stretched. It's the one route where you can see the CW availability wiping out at T-355 pretty much on the dot.