Lots of languages target JavaScript too, but only one got very popular, because ecosystem matters a lot. People are going to have to recreate the current JavaScript ecosystem for each languages, and then probably for each framework in that language.
Elm doesn't have the JS ecosystem, Rust doesn't, Rescript doesn't. The thing is, when you're doing backend stuff, it's usually relatively easy to pull a library with whatever framework you're using. On the frontend, for your components, you need basically a library by framework by language.