Here are just some of the reasons why.
South Africa is seen by the rest of the world as being some what unstable politically and economically. Secondly a foreign business does not want to make a purchase where it is not purely business but politics as well, this is with regard to BEE and AA. Secondly it would take a lot of cash and with fuel prices at the moment every airline needs every single cent they can muster.
If BA (UK) was keen to purchase the airline they would have not sold the % they did to BidVest about 12 months ago.
So no BA have bigger fish to fry.