because window maker is not, and never was, anywhere near the look and (most importantly) FEEL of a real NeXTstep GUI. i have used both for a few years. window maker is a nice windowmanager for X11 but everything else is still standard X11 fare from style of widgets to the behavior. in particular, this matters for how X11 apps interact with the window maker dock.

an actual NeXTstep workspace and dock is quite something different.

NEXTSPACE is interesting specifically because it is not just window maker with some apps that emulate the look.