This must be the reason. Wine seeks to be compatible with a bunch of legacy software, some of which will want to use the equivalent of .NET 1, 2, 3, and 4.x Framework and not just "dotnet core". (Or whatever the new thing is called in Microsoftese this week.)

Edit: maybe this means WPF can be the best way to write Linux applications. After all, Win32 is the stable Linux API... nudge nudge, wink wink. :-D