I'm not a gamedev, but my hypotesis is that one of the reasons is that Mono license at the time was way friendlier than risking having Oracle on your neck. Mod

I'm not a gamedev, but my hypotesis is that one of the reasons is that Mono license at the time was way friendlier than risking having Oracle on your neck. Modern C# also has more features that help avoid allocations, which in turns reduce GC pauses, which are the biggest enemy of a game