First of let me state that this post is very bias towards CakePHP. Truth be told, I haven't even installed or used Ruby on Rails. The closest I've come is looking at various code snippets I've found around. With that said, you may want to stop reading now.
Steep Learning Curve - Laziness
One thing I really hate is learning stuff. It is especially bothersome when you're trying to crank out a project or web application in a limited amount of time. With CakePHP I'm required to learn about the **MVC style of development** as well as CakePHP **conventions**. With Ruby on Rails, I would have to learn MVC, Ruby on Rails conventions and I would have to s**tart from scratch with the Ruby programming language** as well.