Because as long there's a significant "X is fine, why switch?" user base, it will hold back the apps, toolkits, and libraries, many of whose maintainers desperately want to remove the X11 code paths and get on with something modern. We need everybody going all-in on Wayland in order for there to be progress on the modern desktop. Having that crufty old support for an effectively unmaintained display server kicking around is wasting developer effort.