For decades C# was only relevant on Windows, outside Unity never got wide adoption among AAA studios after Unreal became free, and after their license debacle less so, Godot favors C++ and GDScript even with C# support it isn't what most folks reach for, and Microsoft keeps having an adoption (popularity) problem on UNIX culture oriented startups.

While just like Swift on Apple's ecosystem, C# is doing just fine on Microsoft culture environments.

Popularity is relative.