As a contributor to a predating, superior solution, I sure wish that we could have some of Node.JS's popularity. Nevertheless, it makes total sense to me that it's popular.
Here are some reasons why.
This isn't so much a reason as a pre-emption of an argument, that JS is somehow a "toy". It's hard to reasonably make the argument that that is so: billions of dollars have been invested in improving its performance, its reliability, and those investments have been directly leveraged by Node in the form of V8.
That means that there is a HUGE amount of knowledge floating around out there in the industry, in the open source community, in government, in academia... pretty much everywhere.
You can re-use that expertise when you write server-side logic in Node.
It's valuable to be able to share logic across client and server, especially if parts of your application are complex multi-party state machines which can be run in parallel and synchronized between client and server. At the very least it's useful to be able to share declared data structures.
Now, Node JS and Browser JS are slightly different languages, but there's enough of a common subset that you can get a fair amount of work done, especially the logical engine at the core of your application.