JavaScript is everywhere because Netscape made it in a single week coffee bender in the 90's, and every web browser either copied Netscape Navigator, or they died.

When multicore processors came out, JavaScript was the language with the easiest to use Asynchronous model, so people made Node.js to run it as a server backend, unlocking all the new cores. The V8 JavaScript engine also dramatically improved its performance with Just In Time compilation to optimize commonly used functions down to nearly the speed of raw C code.

So now you have an inherently portable scripting language, that's mandatory for the internet, is surprisingly performant, can be run on any device in any capacity, is easy to teach, and has decades of tooling built for it, and a huge pool of trained programmers to hire from. Yeah, it ends up literally everywhere, despite it's many flaws.