They didn't release the frameworks for those OSs.

That's like how Catalysts runtime wasn't backported to older versions of macOS.

They could decouple those frameworks from the OS, but that adds other headaches and you eventually gotta draw the line somewhere so they tend to only add new frameworks with major OS updates.

And given their swift (pun intended) release schedule and fast user uptake of major releases, this is usually not too much of a hassle.

Thanks!