Many of these WW1 BAR belts with the cups never left the USA or got issued. Walking fire was dismissed as a silly concept before the BAR saw combat. And considering that the BAR didn't even see combat until September 1918, it's not surprising that so many of these belts are available in good condition.
Also, even with the recent WW1 Centennial, there are a much smaller number of WW1 collectors than WW2 collectors. Also, not many recent major movies about WW1 compared to WW2. And, even then, the Chauchat was much more widely used and would be a more accurate weapon to show versus the BAR. As of the Armistice, less than half of the US divisions had received the BAR.
Thus, these belts are seen as more of an oddity than a real war memento.