This is a good question. Because it's not like browsers couldn't implement another language if the wanted to.
I think it comes down to a few things. First, JS has a governing body that sets a standard about what the language should support. It's vendor agnostic and has some really smart people on it making good (most of the time) decisions about the language. That would be hard to reproduce.
Second, browsers have a hard enough time keeping performance and implementing features in JS let alone another language.
Third, we've already seen that other compile to JS languages can implement their own spec and then just compile really good, performant JS code that gets run in the browser. JS is just leveraged like assembly with C.