"It was popularized because the Instagram web team used it…" Twenty-five-year front end dev here, I don't recall anyone asking or caring what Instagram was using. Like, obviously projects of that scale make influential suggestions, but when Instagram says "we like This" and AirBnB says "we like That" then you have a couple strong condidates to evaluate.

IMO, the answer is more general: the modern web front-end evolved from lots of different directions. Instead of having one prescribed Correct way (a la AppKit, UIKit, Android Frontend Frameworks, etc.). If Netscape somehow still existed and maintained their peak of influence, things would be different.