A winglet only makes money when at cruise altitude. Therefor, the longer a plane flies at cruise altitude, the more time there is to earn back the investment and additional expenses (extra weight, maintenance etc). A turbo-prop typically flies short hops only, and therefor is not at cruiselevel long enough to warrant the investment.

The 747-400D (domestic) is not fitted with winglets for this very reason as it specifically designed for high density short haul flights.