unclehmm In short: The packaging is focused on it.
When you have a distro with hundreds and thousands of packages optimized towards one toolkit, is it not possible to just strip this one toolkit and replace it with another one, since all the underlying packages - X11, Linux, compiler, and so much more - are still adjusted and packaged towards that first toolkit.
That includes build trigger, versions, patches, and so on.
Every distro can always be only focused on one toolkit and desktop environment, it is virtually impossible with the currently common packaging mechanisms, to provide proper support for all of them, with one single distro.
All the big distros focus on GTK, including openSUSE, Debian and Ubuntu, Fedora and all distros build on it.
Including KDE Neon.