Answer by Ryan Grove:
- It's crazy fast. Node.js (which is where most of the server-side JS buzz is these days) is an incredibly fast evented IO framework that makes it a cinch to write highly scalable network programs.
- Node.js showed up at the right time. PHP, currently the most popular server-side language, has been mired in community squabbling amidst a large effort to refactor its character encoding support, while also losing developer mindshare to the many excellent Ruby web frameworks that have appeared in the last few years. But Ruby web frameworks, and the language itself, have become notorious for having performance issues and sometimes being hard to scale (although some of this perception is based on misinformation), so the appearance of a new highly performant, highly scalable evented IO framework built on top of the world's most popular programming language couldn't have been better timed.