My guess is portability, then obviously performance.

edit: actually there is a specific answer for this particular project - "We had to rewrite the project in C# to add automated code generation (java doesn't have the goto keyword, making automated ASM translation challenging)". There you are.

I mean, that's more or less the reason why it isn't Java, not why it's ultimately C#. My guess is that Java is just what they're most comfortable with, with C# being similar enough but avoiding specific limitations in that case.