Because currently the volume of posts on the subject is probably too low to justify one.
Java is on top for the current rating of programming languages and Servlets, Beans and J2EE for SAP are hot as any other web subject nowadays. Do you still think it is not popular enough to get a dedicated forum?
I still feel that if we have full group of forums for Google, Yahoo and Microsoft then we should have one for Sun :) especially after Sun's step to make most important products open source they really deserve the place.
I do not want to take the name of another forum where I am contributing to the java forum but to tell you we get hundreds of active users in that on daily basis.
But still, you are the more wise people than me so you know better than me if we need a dedicated Java forum here. I just wanted to share my thoughts.