As
@Kuato said, it costs a lot to certify a new airplane (or engine). For GA prop planes they don't sell enough to justify that cost and make it back.
@y_p_w said brand new isn't much different from a decades old model. That's true. In fact the older model often has better performance (payload, speed, efficiency) since it has the same engine & airframe yet is lighter.
Experimental is where it's at for innovation. Most are lighter, faster, more efficient and less expensive than certified airplanes. Yet the difference isn't huge, and comes with greater demands on owner time & effort, and doesn't have quite as good safety record as certified.