There aren't many places in Africa where the demographic is such that a standard U2 show would be feasible. There are few cities between Cairo and Johannesburg where even a modest U2 show would not be outlandish. Theoretically they could play in a city like Dakar, Accra or Nairobi, but the vast majority of attendees would be wealthy expats. Even if they lowered the ticket prices and played at a loss, they'd be facing an audience that was either unsympathetic to Bono's political causes or else had no experience of U2's music or anything like it. I imagine that playing in the Arab world (North Africa) poses political difficulties that could potentially undermine whatever integrity the band has. And South Africa is unlikely to be visited unless the band is on a long-term world tour. Johannesburg was never visited after Popmart, but Asia and the Pacific - Tokyo, Brisbane, Sydney, Melbourne, Auckland - were skipped for several tours too. As much as it would give U2 street cred to play a show in Africa, I don't know that it would serve much purpose in the long run.