Can I make a new programming language based on csharplang

[BoQsc]

The C# specification clearly states that C# language specification is copyrighted.
https://github.com/dotnet/csharplang/blob/main/spec/LICENSE.md

The questions are:

Why csharplang is copyrighted and not in a public domain or under open source license
or public copyright license like Creative Commons.

Why the implemented compiler https://github.com/dotnet/roslyn
is under MIT license but not the csharplang specification?

[YairHalberstadt]

The specification document is under copyright, but the specification as an abstract concept is not. In other words you are free to modify csharp however you like, but you may legally have to write the specification document from scratch yourself, or use the ecma c#5 specification as a starting point.