JavaScript has extremely low barrier to entry. Everyone has a browser, so you can get someone to get started without download any software. That's huge.
Rails has the benefit of providing a framework that can get someone building functional CRUD apps in a tremendously short period of time.
Between those two, you've hit a nice sweet spot where someone can be fully functional as a developer in a short period of time (talent/quality/patience aside).
Maybe the appeal of Ruby is that it's "new" but not so new that it's a waste of time to learn. Also, my impression is that some effort was made to make Ruby's syntax "pretty" in a holistic sense, which might save a teacher some time. Personally, I was turned off by RoR very early on because it was a pain to install on Site5 (the first and only web host to offer it) and then had no compelling reason to try it again after that. Also the concept of "Rails" bothers me, because I think software should be unique and "off the rails" like art. But that's just my opinion :-)