There's this concept of "mature" technologies that have reached more or less the optimum characteristics and are only going to see slight tweaks until requirements or materials change.
eg the assault rifle has seen only one major change since the '40s (the switch to small caliber high velocity in the '60s), transonic passenger and cargo airplane airframe shapes have settled on a single general configuration, &c
The '60s in particular just happened to be a point when lots of aerospace stuff happened to reach maturity, along with our understanding of the common-case aerodynamics.
eg the assault rifle has seen only one major change since the '40s (the switch to small caliber high velocity in the '60s), transonic passenger and cargo airplane airframe shapes have settled on a single general configuration, &c
The '60s in particular just happened to be a point when lots of aerospace stuff happened to reach maturity, along with our understanding of the common-case aerodynamics.