I would imagine that is largely a result of it being as good as unadvertised*, rather than because of a lack of demand. If it were sold online and/or by default to passengers presenting in a wheelchair then sales might be rather higher.A couple of years ago my mother was given a hospital appointment at Moorfields in London and, as she was unable to travel other than in a wheelchair, this was the appropriate discount for her and her carer. There was no way my mother was going to be travelling enough to warrant attempting to buy a railcard. I am pleased to say that her local station had no problems in issuing the tickets with the discount. However, I would also note that during nearly 20 years of involvement with selling tickets at a major London terminal I never sold a ticket with such a discount.
*Yes, it's on NRE if you search for it - but who would? It might as well have been on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying Beware of the Leopard