The point is that they are guest languages on Apple ecosystem and need Apple tooling and languages as means being available.
I may also add that I dislike Microsoft doesn't give to the .NET ecosystem the same care for games developers as Apple does for Swift and existing OS SDKs.
As far as DirectX team is concerned, only C++ exists, and .NET team lets third party folks do the needful.
Had it not been for MonoGame, Unity would never picked C# in first place, gone were the days of Managed DirectX and XNA, when the decision came to be as Unity did their cross-platform rewrite out of OS X.