While better may be a subjective term, it's certainly an objective fact that (as Fenrir remarked) Ruby provides abstractions that make it a more powerful language than PHP. Programming languages are not created equal, and Ruby is way further up the curve towards Lisp than PHP. This is as it should be, since we're comparing apples and oranges.
(BTW, for anyone who has been forced to endure a Java indocrination at university, Paul Graham's essays on the [expressive power of various programming languages and [url=http://www.paulgraham.com/gh.html]great hackers](http://www.paulgraham.com/icad.html) should prove most liberating and entertaining reading.)
I haven't really been keeping tabs, but I'm pretty sure there have been multiple open-source Rails CMS projects started over the past year or so. Check out these links for pointers:
Well, I suspect one reason may be that at the moment, Rails-related work is in very high demand and is lucrative enough that while many people (including me) are certainly creating proprietary CMS systems with Rails, there isn't necessarily enough interest and free time to work on open-source projects for the "greater good".
Personally, I've often entertained the notion of creating an improved Drupal clone in Rails, but as the 48-hour day hasn't been invented yet, there just hasn't been time