I've never run a javascript server in production, but if I do the main reason would be to take advantage of the huge ecosystem. Since js works on both the client and server side, you end up with a lot more programmers hacking on it.

If you check http://www.modulecounts.com/ you can see that npm has as many modules as all the other package repos combined. And it only seems to be gaining momentum. (screenshot: http://imgur.com/a/adqNJ)

Meteor has more stars on GitHub than any other web framework: https://github.com/search?p=3&q=stars%3A%3E1&s=stars&type=Re... Nodejs, socket.io, and express all appear on the first few pages.