The founder of the GNOME project founded Mono to be the “One Desktop Framework To Rule Them All”. It did not work out.

That was to be expected, there was a lot of resistance against using C# because Microsoft tried to destroy Linux in various ways. Toolkit-wise, the Mono on Linux stack used Gtk+. So given the C#-distrust it was easier for people to use C, Perl, Python, or later Vala.