I'm not a developer on Wayland and I haven't looked at the code, so I can't comment directly on their situation. A hypothetical I can come up with is that if they've tried to implement a very secure architecture, they may have painted themselves into a corner if there's no easy way to capture pixels post-composite... Then no one piece of the system has enough authority to see all the pixels to capture an arbitrary screenshot, and oops.

An unsecured screenshot escape hatch can be used by malicious software to scrape information shown in windows on your system, and with modern OCR being what it is (or, hell, a shady Mechanical Turk transcription job), that could be an attack vector for surfacing people's private information.