CSS Modules didn't become the default way to go until sometime in 2016. Before that point people were still stumbling over how to manage their CSS in a component-oriented fashion, and it was only once that happened did React "win".
CS Modules are still not the "default", this space is still up in the air. There's CSS-in-JS (like styled-components or Emotion), Sass, plain CSS, etc.
More important to my point, back in 2015/early 2016 and before, you more likely than not had to somehow manually include the CSS for a given component library into your page. Right about the same time CSS Modules got popular, the CSS from libraries started getting included automatically in the compiled CSS file (whether by way of CSS Modules or something else that at least doesn't conflict with it).
I remember this part of the timeline pretty distinctly because late 2015 is when we started a new project in React, and struggled with how to reliably deal with CSS for several months.
