Red_Avatar: Because of past problems with it. And I've had too many. Stability issues, cross-platform issues, resource problems, backward compatibility problems, etc. etc. And yes, I had it at uni for a year as well. Forgot most of it too, and while it was quite easy to program for, it didn't change my opinion that Java is a bit too rickety for my taste. Maybe it's improved a great deal by now though but I've never had problems with .NET.
Never had any of these issues, it could also be because I didn't really programm very complexe apps with it. The compatibility issues are a thing of the past since javase5.0 and the only cross-platform issues I ever encountered were text encoding compatibility and access control. But if you know a bit about these subject you can easely write around it.
For quick personal projects it's very handy, and comfortable to use. For more serious projects I would go to (pun intended) C or C++ but I would never use something where I have to rely on the Windows platform, cross-platform or nothing is my motto.