The thing is, if you have any sort of front-end that is not entirely server-side rendered, you're going to have to work in JS at least some of the time. If your back-end is also in JS, you now get the benefit of isomorphic code for things that you may want to do on both front-end and back-end.
Then there is also the fact that JS is actually a pretty great language if you know how to avoid the footguns. Granted that's not always easy, but it's a language with lexical closures and easy and familiar syntax, it's also very expressive and has a vast ecosystem supporting it. And you can even add the typescript compiler on top if you want compile-time type-checking.
It's also async out of the box, and while that doesn't solve all problems, it scales surprisingly well with no performance tuning whatsoever, it even has decent asynchronous primitives that make it easier to write correct code.
Yeah; even without frontend code, JS is surprisingly compelling. It's easy to hire for, easy to write reasonably performant IO code in, the footguns are pretty much all well documented and generally understood (rather than requiring knowledge of the deep magic to debug), library support is top notch, and as a language it supports both OO and FP, while being pretty small in scope, terse without encouraging too much code golf.
If you want something mature, with libraries for everything, solid backwards compatibility and basically the best "boring" choice, go with Java. And if your devs want to mess around a bit, mixing some Kotlin in is basically harmless and easy to reverse if needed.
Then there is also the fact that JS is actually a pretty great language if you know how to avoid the footguns. Granted that's not always easy, but it's a language with lexical closures and easy and familiar syntax, it's also very expressive and has a vast ecosystem supporting it. And you can even add the typescript compiler on top if you want compile-time type-checking.
It's also async out of the box, and while that doesn't solve all problems, it scales surprisingly well with no performance tuning whatsoever, it even has decent asynchronous primitives that make it easier to write correct code.