As I understand it, a big part of the problem is that the general aviation market is too small to justify the expense of getting new designs certified. Also, it seems baked into the regulations that the old design was "good enough", so it still is today, even though technology has moved on.

When people are building experimental class piston airplanes with better features, reliability, and performance than the certified world can muster at half the price, you know something has gone wrong.

I believe liability is an issue too. In aviation (as compared with autos) the manufacturer is often found partially liable for an accident, so they can be on the hook even if it was mostly the fault of the operator.