First, for a given power rating, a turbine engine will be a lot lighter than a piston engine. Think about what that does to the aircraft's balance: the nose gets a LOT longer, not to accommodate the hardware put to put the weight where it needs to be. Of course, this only applies to conversions.
Second, a turbine engine will cost a lot more than a piston engine. Smaller turbines cost more per unit of power than large ones, so overall you will be paying more for the same result. The engine might use less fuel over it's lifetime but you still have to pay for it up front.
Third, while turbines in general require less maintenance, when something does have a problem the bill is not for the fainthearted. And you can cook a turbine pretty easily.
So in total it just comes down to money. If you really want one, Soloy sells a conversion kit for a Cessna 206 that will put a 450hp Rolls-Royce turbine in the nose and make you the envy of every other light aircraft owner in the region. The kit starts at $275,000. Plus engine. So you are looking at over half a million to modify an airframe that costs a bit less than that. If you have that much spare cash, pick up a matching MTT Superbike with an only slightly smaller turbine. As Jay Leno said, it's "the best ever motorcycle for shutting up the Harley guys."