I think it's simply that (and this is from my memory only):
Node.js was THE first back-end JS thing, and its "micro" philosophy kind of set the precedent for everything else that follow in that space. It was the time when people started to feel Django/Rails are cumbersome for some of their needs, too, so the MVC approach never caught on for JS devs.
So I think it's just popularity. Pick whatever tool you deem suitable!