People were using Ruby and Python (and before that Perl and PHP) on the backend in many cases. I think it's likely that those are the kinds of projects which are using JS on the backend now, while the Java and C# people are continuing to do their backends in Java and C#.

One of JavaScript's big advantages over Ruby and Python was performance, both because the standard runtime is faster (JITing JavaScript turned out to be a lot easier than JITing Ruby and Python) and because its fundamentally asynchronous nature was a better match for webservers.

And although NPM sucks in a number of ways, I've always found it easier to use than Python's dependency management.