Someone else can tell the story better than I can, but supposedly the cost of certification of new light-aircraft models got out of control in the 1980s, which stifled product evolution, so it became more cost-effective to keep a really old plane airworthy than to scrap it and buy the latest and greatest. I do know there's been a lot of innovation in the LSA (light-sport aircraft) segment, so it does seem odd that four-seater and two-seater evolution would diverge so much.

I have a feeling I'm perpetrating a certain angle to the truth (maybe that product-liability lawyers suck). I'm just passing on what I heard -- please don't shoot the messenger. A more comprehensive retelling would be appreciated.