Because everyone* knows JavaScript. I'd wager that the vast majority of professional software engineers work on the web, or very close to the web, and JavaScript is the lingua franca of the web. I mean just look at the front page of Hacker News on any given week. It's full of talk of JavaScript and new JavaScript libraries and new things to do in JavaScript. When you think about how many more people this can reach without them having to learn yet another language for a very specific thing, it makes sense.
*hyperbole, please forgive me for being not being completely literal.
*hyperbole, please forgive me for being not being completely literal.