Because JS wasn't designed for scientific computing like R and Julia are. Best case scenario is that you reimplement all the libraries Python has, but then you're just replacing Python with another generic scripting language instead of a language built for that purpose. Why would data scientists bother switching to JS when Python already has those libraries, and Julia and R have better numeric and data analysis support baked in?

And if Python, Julia and R don't cut it, then there's no reason to think another scripting language would. Instead you'd be looking at a statically typed and compile language with excellent support for parallelism.