I would say, Productivity. You can easily find a front-end JS developer than say, an RoR or Java developer. And it is pretty easy to train people in Node. I personally seen couple of my team mates picked up Node easily without much effort, but they always fail to understand Java Spring frameworks. This, owing to small learning curve of Node. And JS being both front & back end is one good thing.
A thing to note, is, Node uses event-driven, non-blocking I/O model so make sure to plan your architecture keeping that in mind!