Why would someone use js on a microcontroller?

Could be to draw upon the existing JavaScript ecosystem, or could just be because JavaScript is the language the dev knows best! I feel really nerfed when I have to move from JavaScript to other languages, personally, even if I can get things done in C++ and other languages.

For the same reason someone would use Lua or MicroPython.

If you're moving from a $0.50 to a $1.00 chip due to the overhead of using the higher-level language and save just one $200,000/year dev in the development process, you'd have to sell 400,000 units before the $0.50 chip would save you any money.

Even if you are moving from a $0.50 chip to a $3.00 chip, you can probably still save a significant amount of money on a lot of projects when you put together all the dev-years saved.