.NET was multi-language and C# was an open standard; .NET itself was not multi-platform. I was there at the start. I was a beta tester for the first Visual Studio.NET. The reason Mono exists is because C# is an open standard and Miguel and Xamian (or whatever they were calling themselves then) were able to create .NET from the ground up, called Mono.
https://en.wikipedia.org/wiki/.NET_Framework