Why does a turbo prop loose torque while climbing?

A turboprop engine is no different than any other, it will always produce less power up high due to the decrease in air density. It is suffering just like a piston engine at altitude cause it has to compress its own combustion air too but there is less up high to compress. If you do not move the power lever your airspeed and climb rate will decrease to to the point where you are not climbing anymore and you will be going slow