I think its because recreating rails in javascript just doesn't make sense in the current ecosystem. Either you are rewriting stuff like express from scratch (and why are you doing that?), or you're building on top of those modules and adding configuration conventions and scripts, in which case, congratulations, you've just written Sails.
When Rails first got started, ruby didn't have anything close to the ecosystem that JS enjoys today, so DHH et al. had to write most of the individual components themselves, with the intention of making them all work together.
So in my view, its similar to how Humans and Gorillas evolved from a common ancestor, but you won't see Gorillas becoming Human anytime soon.
