Not Javascript because it's objectively a lousy language. (Imagine if Eich had been allowed to embed Scheme!? What a wonderful world that would be. I'm not going to look if there's a Scheme implemented in JS, I assume there is.) An interesting question is why Elm instead of Scheme-in-JS or Scheme-in-WASM?

The answer is to that is also the answer to "Why Elm?"

Elm is extremely user-friendly, domain-specific language that makes it nigh impossible to write buggy code. You can write code that does the wrong thing, but it will do it correctly. In practice, what this means is that Elm web apps have had zero front-end bugs.

If we allow that end-users and management do not care what system we use to make the website "go", then it's pretty obvious that they could make do with a much cheaper solution than the JS ecosystem. Elm is so simple and easy that anybody can use it to maintain their own web sites or web apps.