KDE, Gnome and others obviously do provide stacking windows, but you do get the impression that writing a stacking window manager/compositor is just extremely hard to do with Wayland. Someone is maintaining a list of compositors[1] and there do seem to be a number of stacking ones, they just don't really get much attention.
The audience of stacking wms is mainly serviced by the desktop environements. Both Gnome and Plasma are bigger than everything else combined.