Like others are mentioning, Javascript is clean, familiar, easy, and has some compelling language features to boot (closures, JSON, etc).

Cultural norms and design patterns in Javascript Land increasingly orbit around projects like jQuery. This sort of influence works out spectacularly-well if you've spent the last several years working countless hours on browser-side code and suddenly stumble on nodejs and its evented/closure style of programming. Perhaps this doesn't describe you, but from my vantage point, this describes a vast number of programmers. There are a lot of people in that demographic who want to be able to rapidly write out web apps in the same frame of mind client and server side and not have to switch to thinking in Python or Ruby.

The fact that V8's engine is competitively fast isn't hurting these efforts either.