My guess is that both are "enterprise" languages with a large ecosystem and pool of users, and both have a fairly pragmatic blend of performance/productivity. It's fairly uncommon to see both used together as they tick many of the same boxes.

It is common in enterprise shops to have native UIs in WPF and the server side in Java with Web APIs.

Now with .NET Core that is slowly changing, but .NET Core still needs to catch up with many .NET Framework features, specially enterprise frameworks that run on top of .NET.

I could have phrased it better. When I want to write something where Java could be a good fit, I use C#. There are times I want to write something where Perl, Javascript, C, or C++ are good fits, but it’s very unusual for me to think “Java or C++ would be a good fit for this project.”