I believe they tried to keep the C# version independently from the framework version, so people can use the new C# version on a old .net framework. But things

I believe they tried to keep the C# version independently from the framework version, so people can use the new C# version on a old .net framework. But things seem impossible for some new features, for example async steam just need new feature from core CLR. So after .C#8, they decide only to support it on .NET core