My hunch is SEO combined with placing less responsibility on the programmer.
The new docs have a page per code symbol, and much more focused single-page topic articles, rather than 20-page programming guides. Docs seem to be structured to answer a question right when you have a problem, but then they don't offer a good way to study in depth before you begin.
The other day a younger teammate asked a group about some API documentation that said the results are undefined if you call this wrong, and had we ever heard of such a thing. And yes, of course, using framework code in unintended ways is always a garbage in, garbage out affair. This made me realize we have come a very long way in eradicating boilerplate and ceremony in code.
There used to be so many things you just had to use the way the docs explained, or else. Doing the boilerplate correctly was a primary focus of code review. Today we have ARC, we don't call alloc separately from init, we never handle KVO without the compiler's help, and we have no IBOutlets to forget to bind. To begin writing for iPhone 15 years ago, I had to study for a week or so with the programming guides, one for the language and another for the platform. Today you can do a one hour tutorial.
So, if you can stumble your way through building an app by typing the period key and seeing what methods are available, and the whole platform SDK exhibits Clarity at the Point of Use, would a newcomer even use the programming guides if they weren't down in the archive?
My best guess is that as macOS aged and became harder to keep backwards-compatible, their dev communication changed from "you must not rely on implementation details" to "you must not know about implementation details".
I don't get it how they messed this up.