JS is the only language integrated into the browser and the DOM. Netscape rejected other languages (Python, Perl, TCL) because of syntax, so Brendan Eich disguised Scheme into Java syntax and hacked up a prototype in 10 days.

Today, Javascript is sooo interwoven with the DOM in browsers that it is near impossible to separate them. Google planned to do this for Dart, but gave up. If they had done this and other browser vendors had followed, adding a third and forth and more languages would have been (relatively) easy, because the abstraction were in place. Then we would have seen a healthy competition.

There is asm.js. Maybe we will get a "web bytecode" at some point, but that will easily take a decade and more. The pressure is increasing, because our CPUs don't get exponentially more powerful anymore. And delivering apps via the web is still going strong.