For a bunch of stupid, hysterical reasons, the Open Source X11 server still has all the warts of early-1990s release. XFree86 used the MIT X.Org core dump to implement open X11 on Linux, but outside of certain new things (DRI, XRender extension) it remained stuck in "world is dumb framebuffers with optional bitter" design.

Over time, a lot of useful stuff was broken and supported less and less, then reimplemented in bad way over compositors, instead of having composting Xserver like Xsgi.

Security extensions or even basic security tools (private input grab) got waylaid because toolkits didn't implement them (try running GTK3 on hardware without 32bit color GL or on system that defaults to lower bit depth than 32/24).

Generally, after letting it get more and more broken, the suggestion for big rewrite arrived. Said suggestion often results in something that resembles "devs first graphic stack", with a lot of basic, important features for daily life relegated to "someone will make an extension for it later" or "use D-bus to implement it elsewhere, and who cares about compatibility".