Divesting from SA meant smaller tax base, fewer jobs and a fall from international prominence.
Divesting from a company/industry means that the stock price may fall. Assuming that the actual profitability of the company remains unchanged all divesting did was make the stock a better buy.
Because people actually stopped buying the products along with not investing in companies. The equivalent with fossil fuel investment would be that people actually stop using fossil fuels, which isn't what happens with divestment.