Because X is not getting much development at this point (personally I still use i3, haven’t switched to Sway, the present works fine for me).
That's X.org, which is controlled by the Free Desktop Foundation.
The OpenBSD people are still working on Xenocara, and it introduces actual security via pledge system calls.
This argument is actually backwards: one of the goals of the wayland project is to draw development away from X. If wayland didn't exist, people would have worked on X11 a lot more.