It worked as an accidental implementation detail in CPython from some other optimization, but it wasn't intentional at the time. Because it wasn't intentional and wasn't part of the spec, that code could be incompatible with other interpreters like pypy or jython.
The reason Guido didn't want 3.6 to guarantee dict ordering was to protect 3.5 projects from mysteriously failing when using code that implicitly relied on 3.6 behaviors (for example, cutting and pasting a snippet from StackOverflow).
He thought that one cycle of "no ordering assumptions" would give a smoother transition. All 3.6 implementations would have dict ordering, but it was safer to not have people rely on it right away.
As someone who just had to backport a fairly large script to support 3.6, I found myself surprised at how much had changed. Dataclasses? Nope. `__future__.annotations`? Nope. `namedtuple.defaults`? Nope.
It's also been frustrating with the lack of tooling support. I mean, I get it – it's hideously EOL'd – but I can't use Poetry, uv, pytest... at least it still has type hints.