The main point of comparison for C# is not Swift but Java, and in Java all the UI frameworks are cross-platform. Also the fact that Mono managed a pretty good implementation of System.Windows.Forms suggests that Microsoft's reasons for omitting it from Rotor were, uh, not entirely technical.
C# and .NET's openness is a great thing and I think it's unreasoanable to expect to be able to write Windows apps on other platforms which lack the underlying APIs to support them. The same is true of Swift - why all the complaining?