Because it's a great programming language, not sure about Mono, but .NET 8+ is now cross-platform (has been since sometime in 2016 actually), so I personally am choosing C# for some of my projects lately as a result.
C# is just a nice all-around language and the runtime is really nice.
By comparison every time I have to look at Java I just want to run away as quickly as possible. All the resources Oracle has, and instead of investing them into making Java a viable platform they shut down the one project that was giving Java an edge in Desktop Development (JavaFX) and basically leave Java in this weird limbo state its in.
It took Java forever to implement lambdas meanwhile C# had them in since .NET 3.5? (2007?)
Microsoft on the other hand, made all of .NET MIT licensed. Even if they change the license 10 years from now, you can pick any of the rich versions prior to use where the license was in fact MIT.
Just my own conjecture, but it's pleasant to write, the NuGet package ecosystem is very well populated, and it combines a great web framework with the ability to drop down into low level code when necessary.
In recent years cross platform support has gotten fairly good as well
I think (and I could be misremembering here) is that Emby started off as C# and both Plex and Jellyfin are forks of it. Also a lot of folks in this sphere seem to be on windows as their primary platform, and C# along with visual studio is pretty on that stack
but there aren't as many IDEs, NAS/homelab manager, backup, games, graphic editing or visualization tools, ... Is it something about codecs availability?
