Probably because people already have their pick of those languages on the server but not on the browser client where it's even more important to get things righ

Probably because people already have their pick of those languages on the server but not on the browser client where it's even more important to get things right from a UX standpoint. Static-typing, immutable datastructures, FP... these are things that even the Javascript ecosystem has been moving towards with React, Typescript, Redux, and more. If you're making a language for the browser, you can offer a lot of value by rolling these into a single tool. For example, it may be much simpler for you to use Elm than to approximate Elm with your own menagerie of JS tools.