[WorldMaker]

They might not port Mono's WinForms to .NET Core? Given how strictly tied to Windows WinForms is, they may not want to give enterprises they are encouraging to use .NET Core any confusion on the subject. Taking that extra step of trying to run the application on Mono shows at least some awareness of possible compatibility issues in the software.

That said, this may still be a great opportunity for .NET Core to encourage trying Mono in its documentation for enterprises looking for cross-platform support. (Hopefully they already knew about Mono by this point, but an extra push won't hurt.