Might it have something to do with the fact that due to flight time, crew duty resting periods and routes/scheduling the airlines have no other role for the airframe and they, or the airport, cannot afford financially or logistically to have the ac at the stand all day.
Not sure how true it is but I heard a rumour that one major airline does't even use a stand at JNB. Instead they get their 300+ pax to use steps to deplane as it keeps their ground costs to a minimum.