I suspect that a lot of the impediment to JITing Ruby and Python effectively was due to large and important libraries (standard and third-party) being written in C using interfaces that were not JIT-friendly. JavaScript in the browser also depends on C/C++ interfaces to important functionality, but the entire stack for each engine was controlled by a single organization, and was always released as a unit (the web browser). I think this largely eliminated library inertia (or whatever you want to call it) as a problem when replacing a JavaScript engine.
The other advantage JavaScript had was that there were large, well-funded organizations (notably Google and Mozilla) competing on performance. Python and Ruby were always community projects and they had a strong emphasis on maintaining backwards compatibility with a large and diverse ecosystem, and there wasn't a lot of demand for faster implementations.
