The reason is basically greed and aviations best old friend: conservatism. Nothing happens in aviation without either legislative enforcement or promise of even more greed. The truth is they have been on a gravy train with turbines and could charge whatever they want. If you're on the type certificate as the only engine certified for an aircraft, well, then the incitement to make them cheap is all gone. They have you over a barrel.
I repost from another thread:
Engineering-wise, there is nothing today that suggests they are hard to make. The tolerances are far lower than in many other industries where none such price hiking appears. Let's not forget they were certified and approved when they were still hand made on lathes and manual mills. Today we have robotic CNC vertical mill stations that have more productivity and much higher tolerances. Neither are the materials used very exciting or expensive.
We could have had a myriad of small turboprops in every LSA and Cessna since the 60's, but they chose to ignore that because the times were good and the military could be relied on to overpay for their stuff - ever wondered why every aerospace manufacturer ever devised has gone after military contracts? One doesn't have to be a genius to figure that one out.
Make a certified 100-300shp turboprop and sell it for the same as a piston engine and you'd see massive sales. If the RC guys at Jetcat, Wren and AMT can make working turbines in their sheds and sell them for $3000, then most certainly RR and the good old boys can make money selling theirs slightly bigger ones for $100K a pop. Yes, I know there are differences between the examples, but they're more similar than they are different.