Because most people are dependent on distributions to provide them with a standardised setup, packages and security updates, and few distros have proven willing to support more than one option for a component as central and dominant as the display server. Sysvinit also works, und unlike Xorg doesn't even have the problem that graphics hardware will eventually leave it behind; and yet, it is now basically impossible to avoid systemd and its many tentacles if you want a mainstream-compatible Linux experience with timely security patches, because its proponents have successfully persuaded every major distro to adopt it and drop everything it replaced. Wayland, unfortunately, is not willing to exist as just another option until it has reached feature parity with X11; rather, it is competing with it for a limited resource (distro support) now, and poised to go in for the killing choke.
