It's not so much that they were unobtanium, but rather that surplus parts are often either feast or famine. These were up for sale on eBay for years, with the prices dropping and dropping, and got as low as $150+shipping. The buyer probably bought an entire lot of them at auction. Then, once everyone who [barely] wanted one had them, they are gone and haven't been seen again since.
The same has been true of the CTIS wheel guards, the step bumpers with "barn door" tailgates, hydraulic jack pads for the axles, FMTV seal drivers, and many other parts.
My personal takeaway: When you see something neat that you haven't seen before, buy it. You may not get another chance.