This is nothing more than a clever ploy by Microsoft to regain control of C# from the community, becoming the gatekeeper of the new ".NET Standard" library... rather than opening up some of their coveted, proprietary codebase. (Winforms)
TL;DR Micro$oft being Micro$oft
We highly care about making it easy to write code that works cross-plaform, especially on Linux. That's why we created .NET Standard!
Xamarin has an abstraction layer for UI called Xamarin Forms. This will allow you share UI code across .NET Framework, UWP, iOS, and Android.
What makes you think that open sourcing WinForms would be any good? It's probably tied to Windows to the gills.
If anything, they should be doing something else, they should create or even better, support an existing UI toolkit and promote that as their cross platform UI.