Basically a long time ago Linux/Unix was run on big machines in a separate room with all the fancy graphics hardware, and you’d have a dumb little machine at your desk that could barely draw pixels on a screen. So X11 was designed with all these fantastic neat server-client mechanisms that made it great for running on a mainframe.
Fast forward 30 years and all that stuff is useless now that everyone has built in graphics (as well as several other issues with X11’s archaic design). So some smart people who didn’t know any better made a new thing that everything has to be rewritten for (because they were smart, but didn’t know any better). Then someone who did know a little better was like, what if we take the old bloated one and rewrite it for the new lean one. So now everything runs in an X11 session inside a Wayland server, which has to be rewritten for everything because Wayland is a protocol, not a server.
But one of the really nice things about it is that everything has to be rewritten, so we can make newer, fancier bugs.
Edit: I don’t want you to take the impression that I think Wayland is bad. Wayland is way better than X, it just sucks that we have to rewrite a bunch of stuff for it and figure out new ways of doing things that were dead simple in X, but very insecure.