Do developers really care whether a tool is written in JS rather than Ruby, or vice-versa? If you're running *nix it's pretty likely you have Ruby installed out of the box anyway.
There's nothing inherently tying you to Ruby anyhow, anyone could write a SASS parser in JavaScript. It's just that nobody has because there is no need for it.
It's more convenient to be able to handle all dependencies through the same package manager.
Personally I prefer Scala and thus use SBT for most dependencies, requiring the user to use Bundler or NPM as well for just one dependency can get a bit annoying. In the case of SBT it actually does have some integration with NPM since it's used so much for client assets (and there is Trireme which is a pure-Java Node clone), but Ruby dependencies still mean adding one or two manual build steps.
Node-sass isn't a JavaScript port of SASS, it's a Node.JS binding for libsass, the native (C) SASS compiler. The first paragraph of the readme tells you as much.
There's nothing inherently tying you to Ruby anyhow, anyone could write a SASS parser in JavaScript. It's just that nobody has because there is no need for it.