Because coding Javascript with support for multiple browsers in itself is already a hell. Sure, we have cross-browser solutions and polyfills, but this only abstracts the developer from all what's going under the hood. Now say we throw in a new browser scripting language (like what we've had over the years), developers would have to support multibrowser, multilanguage environments, and I can only imagine how exponentially difficult that could get. On top of that, the end user doesn't even care. All they need is consistent experience with their favorite browser - they stay in for the experience, not the technology.