It's a whole different ecosystem - it's not like Scala where I can sling in one class on an existing Java codebase. .net isn't just its own VM, it's its own IDE, its own build tool, its own packaging model....

For little standalone pieces - the kind that people write in D or OCaml because they want to - I can see .net being used on linux. But I don't see anyone migrating an enterprise Java codebase - it's just too much effort for too little gain.