A chat application is to node.js what a blog was to Ruby on Rails or PHP -- it's basically the default example of every framework and library. Low complexity, high throughput. So you'd have no trouble finding good examples and libraries.

For a while, Erlang was quite popular in that area, but as some orgs only used it for that and thus didn't get too deep in the tech stack, they were eager to switch to something they imagine they know a bit better.

The only programming language we all know is JavaScript, so it was not an option to go with another backend language.

I would love to learn Elixir though, from what I have heard it is great for real-time!