> I really have no other plausible explanation...
I have a better one: They designed for embedded devices first, desktops second or not at all. The first users of Wayland were things like vehicle infotainment systems, which operate under a limited mobile-like "one app at a time" system. Back then Ubuntu Touch (using Canonical's Mir) also looked like a prospective usecase for a new display system.
As far as I remember, the Wayland devs' approach to desktop support was "we made the basic protocol, now you desktop people either integrate functionality into the compositor (what GNOME did) or make your own extensions (what wlroots, KDE did), we don't care". With that kind of "leadership", no wonder it took years to reach even basic consensus.