It’s probably a function of human nature to be conservative there.

It seems that the FAA is trying to optimize for the fewest unknown unknowns, and until the 737 MAX it would be hard to argue that entirely new airframes, propulsion and control systems operating in flight regimes that have only been done one other time (intentionally anyway) in commercial aviation would achieve that objective better than incremental changes.

The associated bureaucracy bloat can be a feature because it’s harder to sustain a ruse over time.

That said, it really does impede development of arguably safer systems.