why are turboprop/turboshaft engines rated only in horsepower and never in ft-lbs of torque like diesel engines or electric motors?
The reason a Diesel engine has a torque figure and a horsepower power figure is because max torque and max horsepower occur at different engine RPMs. Have a look at any performance curves of a piston engine and this will be evident. Hence, to define in a few numbers the maximum performance of the engine, you need to give both figures.

However, turboprop engines for propeller aircraft, and turboshaft for helicopters, operate differently. Maximum efficiency for a rotor or propeller occurs at a certain RPM. Operating at different RPMs is inefficient. Hence, propellers and helicopter rotors are designed to operate at a fixed RPM. As load changes, the blade pitch angle is changed so it “bites” into more or less air. Hence, RPM can be kept constant, despite the load (and hence torque demand) changing. Now, since the power turbine is running at a constant fixed RPM, only 1 performance figure is needed (SHP or TQ) because the other can simply be derived from it.

Torque is just as important in a turboshaft or turboprop as a piston engine. These engines all have a torque sensor that usually works by measuring the twist in the power turbine shaft. Most helicopters do an engine performance check daily that involves reading TQ of the gauge prior to flight to ensure adequate performance.