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.

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.”