In particular, the Rails-y way of doing things requires particular concise classical OO abstractions that are difficult to provide in a pure prototypal JavaScript way without introducing your own class library (which there are far, far too many of).

For example, if you look at the definition of the App.User here: http://towerjs.org/models, trying to do the same kind of thing where you can quickly declare a class that extends a base class and adding things to it can become quite awkward.

You'd have to manually construct the prototype chain (or use the library-specific class DSL) and have to repeat the class name in order to call the "field" method there.

TL;DR Rails uses mixins and classical OO extensively, CoffeeScript facilitates this, JavaScript makes it difficult.