Its a valid question. Seems that it is a Javascript interface Written in Python that parasitically ties into existing executables. Why not write the whole thing in JS?
Frida's core is written in C and injects Google's V8 engine into the target processes, where your JS gets executed with full access to memory, hooking functions and even calling native functions inside the process. There's a bi-directional communication channel that is used to talk between your app (Python?) and the JS running inside the target process.
On top of this C core there are multiple language bindings (Python, .NET and a browser plugin), and it is very easy to build further bindings for other languages and environments (Node.js could be a future binding if anyone's interested in helping out with that).
I was thinking this exactly, it would be great to have it in node.js. I'd love to dedicate the time, but I've got my own open source project I'm trying to get going at the moment.
For anybody that does want to try this before the node-bindings are available, maybe package it with node-python? https://npmjs.org/package/node-python