The important aspect of SwiftUI is tight integration with AppKit and UIKit. SwiftUI for Win32 (or whatever) would be a completely different beast.
There's nothing stoping anyone from implementing such a thing, just as someone wrote a SwiftUI for HTML. All the pieces are there, but it would be a huge amount of work unrelated to SwiftUI on Mac and iOS.
It seems like it could still be coming. They might be following the same pattern for SwiftUI (and Combine) that they used with Swift itself:
1. Create and introduce it as proprietary
2. Iterate fast, changing a lot of things to improve it quickly.
3. Stablize
4. Open source it (and continue iterating with an open process)
The advantage to starting proprietary is the initial iteration phase can move move quickly and stay focused on the goals of the people in the controlling group.
Not only did they announce that it would be open source immediately, unlike the source code dump they did with WebKit, Apple made the entire git commit history public from the initial commit.
FWIW, WebKit's complete history is available in its Subversion repository, all the way back to the initial commits in 2001 when they were importing KHTML and convincing it to compile without Qt. Granted, there was around two years after Safari's initial release where Apple simply published source code drops, but the full code history has been public (first in CVS, then in SVN) since mid-2005.
Where? Here's the transcript of the initial announcement [1]. No mention of open source that I see.
Dr. Dobb's had an article in 2014 [2] which said "Swift is proprietary and closed: It is entirely controlled by Apple and there is no open source implementation."
EDIT: I found a HN post from 2015, just over a year after Swift was released, titled "Swift will be open source later this year" [3]. Not immediate at all.
Not "almost immediately". They announced it would be open source at WWDC 2015, a year after Swift was first introduced at WWDC 2014. The actual open source release then happened a few months later in December 2015.
XNU is one of the few parts of macOS that is actually open source. I’d be pretty surprised though if swiftUI were deeply dependent on the kernel itself, rather than on some graphics toolkit within macOS
It seems pretty clear they have 0 plans on doing that. They could've open sourced it in the first place, along with Combine.
Not open sourcing Combine seems like a pretty short sighted decision to me.