I think it is just that there are so few of them, I know some I have did google search for revealed nothing, I can't locate them, and they don't have to be old either. It is just as hard to find one that is too new, or one that maybe just didn't get popular for some reason. I think a catchy name automatically makes a plant 'wanted' but if a plant has a 'boring person's name', it is not as popular. A lot of the ones that really took off had names that rhymed, or were kind of funny or unusual or food names, ect. That is just my 2 cents, I know some may disagree. I think some are registered but then don't take off well -don't grow well or don't grow well in different zones so the availability fizzles out as others come available.
Advertising may have a lot to do w it as well, and the bedside manners of the hybridizer. I know there have been times I have tried to contact a hybridizer or a business only to be 1. ignored, or 2. we are so busy we don't need your money... or other similar effects. Well then I buy something different somewhere else. I guess a lot of reasons. There are some nurseries that specialize in the older varieties more-so. I like them myself but sometimes they are hard to find.