The problem Javascript-to-javascript compilation and even minification of Javascript is that it makes debugging very difficult. A byte-code with embedded debug information (such as line numbers) would make compilation a much better option. And then you also get minification for free.

There are advantages to using other languages besides JavaScript. Many of things currently being done by libraries and boiler-plate code could be handled by a compiler (e.g. CoffeeScript). And libraries like jQuery might be less necessary if your compiler handled the cross-browser issues.