IMHO, the core piece is that supporting a language is very, very expensive: tooling, CI, linting--especially standards. Many of things that ensure quality are not language agnostic. It's not about learning a language, it's about maintaining one.

If you are a web shop you have javascript. Using node.js allows you to eliminate one extra language.

Is it perfect? Definitely not. Is it best for everything? Probably not the best for anything. But, it's good enough for most stuff. It doesn't make sense to incur the burden of another language for a one off task.