For me (5) is the major reason. Web development with Rails/Django is so much easier and smooth compared to that of any Java/JVM based frameworks, it doesn't make sense to use the Java/JVM ones. Even a Ruby/Python novice can pick up and run with Rails/Django. You will end up building a robust, secure solution following well documented best practices and leveraging abundant 3rd party library ecosystem.

I think Play Framework is trying to achieve such usability, but I don't think it's there yet.