My personal, completely-unsupported theory is it was a combination of 3 qualities.
(1) New types of things, while unlike anything that came before, were still simple and understandable enough that someone without formal engineering training could understand their use and offer improvements.
(2) Engineering was still seen as something that was approachable by anyone, and so more people availed themselves of the design tools it presented.
(3) Manufacturing was physically colocated with design, increasing agility to implement improvements.
Since then, we've moved into geographically disparate manufacturing of such optimized and tightly-packaged systems that as simple of an ask as "Could that light be red instead of green?" requires overwhelming machinations to design and implement, resulting in "Let's just leave it green." (Repeat for every UX component of a system)