Calamares uses the QML / Qt toolkit. Most of the people involved in Calamares are also involved in the KDE Project.

XFCE use the GTK toolkit.

So, it is totally reasonable to say that KDE is “more native”.

While Wayland maybe a factor, KDE itself will not be fully Wayland compatible until Plasma 6 next year. So that does not really explain the timing of this move.

I use XFCE myself so I am a bit nervous about the change. We will see.