On older turboprops the added complexity of the prop control along side the engine control is a factor.
On newer types with FADEC etc it tends to be the use of common type ratings eg the Dash 8 -400 has FADEC so could easily have been fitted with autothrottles (from a technical perspective), the 100 series didn't so the Q400 can't either (from a certification point of view or it would have been a Dash 9)