Says who?!?
I think the real reason could just be pricing, producing plastic breifs in the near future will become very expensive and getting plastic-backed breifs will be harder. It sucks, but at least plastic pants will be practical, probably for a long time to come. As long as there's a market (which there certainly is) and it is possible, there will be plastic-backed breifs around, for sure. They could get annoyingly expensive, however. We'll have to see.
I can't see why they would not want to sell them. After all, profit is profit and they are medically relevant for plenty of people. Pharmacies stock all kinds of irrelevant crap as it is, so I can't see how that could be a problem.