X11, i.e. some of its implementations, don't expose all internals, and clients don't just plug into any function in the binary. The X11 protocol and some of its extensions clearly define how clients can communicate with each other or the server.
Wayland on the other hand does not. How and if clients can communicate with each other is considered out of the scope of the Wayland protocol, except a few exceptions (e.g. copy/paste) so it's up to the Wayland compositor. That's why there are already multiple different "APIs" to do screenshots on Wayland compositors, some more powerful some less, and not a single one is implemented by all major compositors - basically fragmentation hell. That's also why stuff like color pickers still don't work without tedious hacks (e.g. misusing some of the screenshot APIs).
Wayland on the other hand does not. How and if clients can communicate with each other is considered out of the scope of the Wayland protocol, except a few exceptions (e.g. copy/paste) so it's up to the Wayland compositor. That's why there are already multiple different "APIs" to do screenshots on Wayland compositors, some more powerful some less, and not a single one is implemented by all major compositors - basically fragmentation hell. That's also why stuff like color pickers still don't work without tedious hacks (e.g. misusing some of the screenshot APIs).