In simple terms, the crew had set their altimeters to a datum which indicated altitude above mean sea level (QNH), rather than the datum which indicates height above touchdown(QFE).
Due to the terrain elevation, they were much closer to the ground than they thought they were; descending to a cleared height above touchdown with the wrong datum meant that the aircraft flew into the ground under control.
In those days, neither radio altimeters nor ground proximity warning systems (GPWS) were fitted to such aircraft.
Nowadays, the approach is flown using QNH and GPWS is a mandatory requirement. The controller will also have a direct indication of the aircraft's altitude from secondary surveillance radar information transmitted automatically by the aircraft.