The answer is pretty obvious: because you want to use .Net on Linux.
This of course begs the question (which I think is really what you’re getting at): why would you want to use .Net on Linux (over Java)?
Lots of reasons:
- Common code between your server and, say, a WPF or Winforms app;
- Use of a particular .Net language, like F#;
- Language features that aren’t in Java (closures, operator overloading, partial classes, runtime generics, indexers, delegates, LINQ, var types, etc etc etc);
- Your skills or those of your team are already in C#;
- etc.