1) A lot of embedded projects have HTTP interfaces for command & control. So that means you usually have somebody on the team who knows HTML & CSS & the giant pile of tech that is web dev these days. Which means they probably are more familiar with JS than C or any other more typical embedded scripting language like Lua.

2) C needs to be compiled and often flashed onto the device to run, sending new JS over the wire to execute is a lot more convenient, especially if you want customers to be able to customize their devices.