The XDR is a week or two more backordered than the Mac Pro (mid January). Diverting hundreds to stores so folks can check instagram or ticktock or macrumors at the store shouldn't really be a priority at the moment. There are few folks that will come in and look at for what it is but most will treat it like a generic monitor.
At stores where there is no Mac Pro on display, doubtful they'd be sending either of these to that store. There are some stores in some targeted areas that may get one, but it will probably need some more time to trickle out. A limited number of stores have a business customer meeting area.
Besides most stores are gearing up adjust the layout for the volume holiday sales for lower priced products. ( the Mac Pro has usually been the one bumped out for the Holidays even when on display). An odd time of year to soak up tons of display space on something that relatively unlikely to sell in most locations.
As the other response more than few folks with "try before buy". Apple could just ship unblemished returns to stores as they come back since can't sell them as new anymore. (or if folks are close enough, just drop it off at the store for a return. ).