Because it has bugs that make it unusable for some people (e.g developers) - https://bugzilla.mozilla.org/show_bug.cgi?id=1628162
104 karma · joined November 26, 2014
Actually, there are not much really complex plugins for editors; most of plugins it is 1-5 python files that require no dependecies.
Also, I don't think that it is a good idea to implement CPU intensive real-time applications in javascript (e.g. code parsing or syntax highlighting). There are plenty of languages/tools that suit well for it, there was no reason to pick a javascript as the primary language for a editor.
I agree that language aware refactoring is always better and precise. The pity is that it is not available for most languages.
All plugins are [almost] language independent and are designed to use with any language.