Weston was only ever intended to be an example, and its monolithic nature meant that it wasn't particularly useful as a platform on which others could build (and this was even more true early on, before libweston).

As a result, GNOME and KDE both did their own implementations - and from that seed grew a host of complaints about things not working in one or the other, when on xorg they had worked more or less the same. The lack of a common entry point for "plumbing" also hurt, and can probably take much of the blame for the initial pain that many faced when first moving to a wayland-based DE.

But, of course, that's only obvious in retrospect. I don't think it was at all clear at the time those decisions were being made originally - in other words, it was a mistake rather than malice.