There are turbines allright but the SFC is ~50% of the piston engines, so the "business case" hangs on flying at FL250+ to get the TAS gain, which limits the mission profile considerably (for traditional GA flying).
I don't want to start another twin v. single thread but there are evidently loads of people who don't attach enough value to the SE risk to pay the extra for the 2nd motor. Or perhaps they looked at the stats and found the PT6 ones rather good.... better than pistons twins for sure.