Jsmitty:
During the take-off phase of flight, the aircraft has low kinetic energy and low potential energy. Hence the benefit of the autofeather system promptly identifying and feathering an engine following a loss of power.
During the approach and landing phase of flight, the aircraft has both kinetic and potential energy in reserve. An engine failure is less of a problem because the total power needed for approach and landing is less than what one engine alone can produce. For that reason, there is no need to use autofeather during that phase of flight.
There are other technical issues involved (for example, power lever position is steady during take-off, up at the forward end of the range, but variable during approach and landing), however; then issue of energy reserves (both potential and kinetic) is the main reason.
Michael