There doesn't seem to be as much turnkey rails stuff in use as there is PHP (or perl) stuff across the board. Other examples would be phpBB or phpMyAdmin and vBulletin.
I wonder if part of the gap is perhaps cultural. Rails comes out of 37 Signals who made lots of money doing Saas so they are going to be something that rails developers naturally aspire to.
In an instance where a PHP developer might say "I can make this software, I'll put it up as OSS or I'll make it commercial and charge a fee to download it" a rails developer might think "I'll stick it up on my own servers only and charge people a monthly fee to use it or have an ad supported model".
In other words , rails people may just be a bit more entrepreneurial. PHP also grew up in an age where you would probably be considered a bit crazy to do all your project management on the web.
Aside from PHP being around twice as long and around for years during the nascent phase of open source web apps, the number one reason it has more and more popular turnkey apps is the same reason it "killed" perl: easy deployment.
The secondary reason is indeed cultural, but not it's nothing so abstract as what 37signals does. Rather it's the technical culture of moving fast and breaking things. With Rails you need to stay on top of upgrades all the time. If you are actively working on an app then this is a net benefit because you get new features. But it also creates maintenance work downstream, and not just with application code, but application servers are also relatively unstable. When you have something with a high heterogenous installation count like WordPress or other popular open-source apps, the pain of maintaining version compatibility and keeping it running over time far dwarfs any benefit of using a more powerful language like Ruby.
You would still have an update problem with wordpress since it's a very popular attack target. One of the things that has made me shy away from using it is fear of running into a problem like "we need to upgrade to version X in order to fix a security issue, but version X breaks plugin Y".
I guess what I was mainly getting at was that prior to around the time that rails and 37s got going the idea of paying monthly for web based Saas hadn't really exploded in the public consciousness (of course this was due to many factors other than 37s and rails).
So web developers who didn't want to do consulting would naturally think of the shrinkwrap type model first as a way of selling software.
I wonder if part of the gap is perhaps cultural. Rails comes out of 37 Signals who made lots of money doing Saas so they are going to be something that rails developers naturally aspire to.
In an instance where a PHP developer might say "I can make this software, I'll put it up as OSS or I'll make it commercial and charge a fee to download it" a rails developer might think "I'll stick it up on my own servers only and charge people a monthly fee to use it or have an ad supported model".
In other words , rails people may just be a bit more entrepreneurial. PHP also grew up in an age where you would probably be considered a bit crazy to do all your project management on the web.