The difference being that no one in their right mind is thinking of rewriting a browser in Java to also make it faster, while that's exactly what Servo/Stylo etc are all about.
For what it's worth, Python was also considered at some point for use in the Firefox codebase. I don't remember the rationale for not adopting it, but I think the idea was "we all like Python, but we already have one messy language (JavaScript), let's not make it two".