It is the cost of the unused copper landline you need to buy in SA to run the ADSL. With fibre, it is easy to take out a pay-as-you-go VoIP phone account and port (keep) your old Telkom landline with that new service. Many don't realise this and still cling to ADSL and their phone number. Fibre is also a lot more stable than ADSL, so if fibre goes into your area your biggest task is choosing an ISP to go with... hint: think about good customer experience and flexibility to change your package (do not go on price alone).
#ADSL#^Fibre is much cheaper than ADSL in South Africa – Here is the simple reason
Uncapped fibre-to-the-home is now much cheaper than comparable ADSL or VDSL services in South Africa.