You can just as easily say RoR is slower (than, say, Symfony 2.0 on PHP with an opcode cache) and harder to work with (from a system administration perspective). Take a look down the top 100 website list, and of those sites that are web applications, more of them will be built on PHP than not. I don't think there's even a Ruby application among the top 100 sites except parts of Twitter, but I could be forgetting some…
Also, hiring PHP developers is easier than developers. There are just more people with PHP knowledge. Not saying that is a good reason to use a language, but it is a factor.