Multiple reasons...
First, I'm on GNU/Linux (Gentoo distro), and I do most of my work from the terminal, so I use vim for source editing. I use eclim, ctags, and omnicomplete. C# (mono) does not have have an equivalent for eclimd to my knowledge. And ctags and omnicomplete don't work well, if at all, with C#.
Second, Mono is a horrible implementation with multiple issues. Ever tried using path variables? Or attempted to load a resource from a tmpfs system? And then spent a week attempting to debug the issue only to find it was a bug in Mono? This isn't to say Java is perfect, but it tends to work much better for GNU/Linux systems.
Third, Java has a far superior build process (
https://maven.apache.org/ although technically this is language agnostic, I had a lot of issues getting to work with C#), so I can wrap anything in a bash script or a Makefile, and run it directly from vim. It is also much easier to include external libraries since Maven has a central repository. Apache also has a much better library for dealing with asynchronous network IO (almost as fast as C/C++).
Finally, I plan to use Python for scripting, and Java allows a far more seamless integration (Jython).
There's a ton of other reasons, but these are the main ones. I would honestly prefer C++ if I had more time.
Anyway, I hope my really long post answers your question
