Unity used to have a JS interface, where one could either write their game object components in JS or C#. They deprecated the JS interface.

I imagine it was too much work to support two languages? As a dev reading docs, I used to have to deconflict the parts of the docs talking about JS from C# (not difficult as an experienced dev -- I wonder how the first timers fared). I'd also have to translate advice from JS Unity programmers into C# when researching how to do certain game things. I'm sure JS Unity devs had to do the same with C# advice.

I'm happy they aligned on one language. If JS had an optional type system baked into the language, I think it would be a better choice. However; it doesn't.