1. npm is extremely easy to use and has a lot of traction. Language X is going to be a tough sell if it lacks good libraries.
2. node.js devs tend to be religiously adherent to the idea of small modules. While not everyone like this, and some would prefer large frameworks, it is definitely a more flexible model.
3. Speed. I happen to write mapping and geospatial analysis software. Most of the existing solutions are python based. I spent months optimizing one particular algorithm in python and was able to get processing time down to 70 seconds. This was not acceptable for a web app, so I wrote a new geostatistical engine from scratch in node.js. New operating time: 35ms. Sure, there were probably ways to optimize the python code further, but it was never going to touch the node implementation.
https://github.com/morganherlocker/turf
4. In data analysis type applications like this, async IO is a huge help. More connections with less servers means huge cost savings.
5. Syntax is just syntax. When I was a python dev I would look at javascript and cringe, now I find myself doing the reverse. I fully expect to look back at both and cringe when I am a few years into my next language. We all experience that feeling when we see something different than we are used to.