I would think because as all but one flight had approached Antarctica in brilliantly clear conditions and the final letdown was entirely VFR with no need for instrument cloud break procedures
It is not known what the conditions were for previous flights to McMurdo, an example being an aircraft observed operating in and out of cloud at low altitude whilst reporting being in VMC. In any event, a VMC letdown prior to 8th Nov '79 had to be made in the proscribed sector over McMurdo with radar monitoring, obviously never carried out because the airline would then have become aware that it wasn't possible for the radar to monitor descent. Following the removal of the radar monitoring requirement after 8th Nov '79 the descent was still required to be made in the proscribed sector centred on McMurdo, that sector being an area from 120° grid through 360° grid to 270° grid centred on McMurdo Field and within 20nm of the TACAN, and absolutely no descent below 6,000 was permitted. The fact that it may have been gin clear doesn't pass muster, you can't be just a little bit pregnant when it comes to abiding by SOP's, as evident by the final flight. In fact no evidence of permission to operate outside the sector below LSALT is available, other than 10,000 when north bound up the coast on departure from McMurdo to Cape Hallett.