It's that Wayland's design, implementation, their attitude, and everything else about it is terrible. It could have been implemented without compromising on features or convenience by explicitly specifying minimalistic controlled side channels in their security model from the start, instead of shifting it onto ad-hoc implementations. And of course the windowing system is already too large of an attack surface. Many people are thinking about going full Qubes due to the current realities, while the others live in denial and call even window isolation "paranoia". Fascinating.
Apple apologists keep making the excuse that Apple has to provide no side loading because if there was any single way to do it, all scammers would be making all grandmas do that. They're correct.