As a disclaimer: I really like Wayland and use it as my daily driver for months now with KDE/Proton.
Now my answer, based on my best knowledge:
Because there is no real Wayland to implement, the base Wayland protocols are extremely bare bone and most of the heavy lifting is done by all the different wayland compositors like hyprland, proton, Mutter, weston, wlroots, gamescope so as a developer you don’t have one target to program against (X11) but lots of different wayland implementations and those are not always doing things the same way or providing the identical interfaces/API or have the same level of features.
On my system is at least one wayland only program that works absolutely fine when started in a wlroots environment but crashes (reproduceable on different systems) with a segmentation fault in Mutter or Proton.