1) Open source projects like this would tend to want as many possible contributors as possible. There's more people out there who are comfortable with plain JS than with CoffeeScript.

2) One may prefer a more explicit language, filled with semicolons, curly braces, and "function" functions.

That said, I personally would have greatly preferred it as CoffeeScript, which I use regularly on projects where the people I work with have the same preference.