Possibly because people's opinions on them are formed by the kind of comments you get on here, without ever having seen one work...
Compared to the numbers of patterdales, russells, lakies etc being worked, plummers are amongst a minority to start with, so the numbers of them being worked to ground is bound to be far less again. I know a few people who do work them to ground, most of whom don't come on forums and post about it.
The sire of my pup pictured earlier, sadly no longer alive.