.NET is big for mobile app development, particularly when you include Unity. It's really one of the best solutions for writing Android + iOS apps, where you can share a significant amount of code and still have native-looking apps. You don't need to depend on clunky electron which makes your apps huge. .NET apps are smaller and faster than electron based apps. Swift is probably a common choice these days, but wasn't an option a few years ago.
Unity is used by a huge number of mobile games and uses C#. The Godot engine also supports C#.
On desktops not as much, but it is sometimes used for cross platform development, and plenty of games on Steam are written in Unity and run cross platform.
Linux desktop could have had a better story when MonoDevelop was gaining popularity, but Linux/desktop support basically got sidelined when Xamarin shifted focus to mobile development, and the acquisition by MS killed mono.
Since MS went fully open source on .NET, the situation for Linux is improving, but it's not yet a choice for most developers on Linux and may likely never be, because everyone wants to write web apps.
Many of the Linux desktop apps written with Mono have been abandoned or are barely maintained.
Even those not using containers, Linux website hosts are generally cheaper (including on most cloud providers). In addition to new apps I've seen some older web apps pressured into .NET upgrades just to have Linux runtime support to run on cheaper servers.
