Interesting fact, when they forked the CLR, they actually started with the Silverlight code base. See https://channel9.msdn.com/Blogs/dotnet/NET-Foundations-2015-... (somewhere near the beginning)

I guess it makes sense. Before .NET Core, Silverlight was the most x-plat version of the Microsoft .NET Runtime(s) (excluding Xamarin/Mono), so it would've been a good starting point.