I wouldn't wish to defend them, but by all accounts the biggest reason is because Xorg's code quality is quite poor, which is (if true) somewhat fair. I wish they tried replacing it without breaking everything and shipping the terrible design that is Wayland, but the goal of replacing Xorg is reasonable.
https://github.com/X11Libre/xserver
The reality is much simpler. The parent corporations, mostly Red Hat really back in the days, wanted to have more control. And, in some ways, wayland is indeed simpler as a protocol.
Also, you write "replacing Xorg is reasonable". First, I don't think it is, but even more importantly than that, wayland is NOT a replacement. It is another display server and protocol. They do not have as goal to be compatible to Xorg at all, so "replacing" is the wrong term here. They indeed broke a ton of things, or rather, required of downstream applications to be rewritten - but the statement that they aim to replace Xorg is not the case. They are simply another display server + protocol at the end of the day. And they won't be a full "replacement" feature-wise either, that's why using wayland always feels like a dumbed down variant of xorg-server. Admittedly wayland also has a few new features such as the easy restart, but this may come to the new xserver too eventually. One has to wait and see. (The easy restart variant is something I like in wayland; hopefully xserver will have that too eventually. It would be technically possible. Also, xserver already is different to xorg-server, in many ways better too. Neither wayland nor xserver aim for 100% compatibility with the old, venerable xorg-server anymore.)