For many reasons, namely that we've already solved most of these "problems" years ago with simple concepts like OOCSS, SMACSS, and BEM. I suggest taking a look at this rebuttal. http://keithjgrant.com/posts/against-css-in-js.html

The reasons people are running into problems with global css scope is because they don't understand the basics of how to write effective and maintainable CSS. Seems like many front end devs nowadays grew up with css hand-holding libraries like bootstrap, and can't seem to wrap their heads around completely necessary things like taking account for CSS specificity and how the cascade works and how to use it to your advantage.