Because your system may require that you can change code at runtime without reflashing your micro. I very, very much dislike JavaScript, but it's not an outlandish requirement and there's not a lot of (what I would consider) great options in that arena with expansive C interop (which you would need for a "real" application). There's basically JavaScript, Python, and Lua. There's plenty of other stuff out there, but none with the same amount of backing as those three.

Embedded development is the bat country of software. You will definitely have to do some unsavory things to get where you're trying to go.