I think that C-Dory's are great boats. However, they are not very flashy. They don't look like they would go that fast ('though 30mph is fast enough for me) and the interior is quite utilitarian. Compared to a showroom 25' boat with a cabin the C-Dory is pretty dull inside. Without knowing anything about boats the average person when seeing a C-Dory next to a Bayliner, Tahoe, Sea-Ray, etc., is likely to pick the nice shiny one.
People who have been around boats for awhile realize that a lot of the flashiness of the chlorox bottle boats makes for maintenance headaches. But they probably figured this out after living with a Bayliner class boat for some time.
So, a lot of people buy the flashy boats. This allows for economy in the production. This also means that there is a relative glut of used boats available which drives down the price of the used boats.
One the other hand, C-Dory's are built one (or a few) at a time. A lot if it by hand. You can tell the factory you want this or that changed and they'll do it for you (try that with one of the major manufacturers of similar sized boats). People who know also realize that the simplicity of the interior and systems means a lot less maintenance and the ability to easily customize the boat to suit their needs.
However, the number of people who appreciate these differences appear to be fewer in number than you might expect.
C-Dory could probably increase the sales of their boats if they tart them up a bit. However, that would start to negate the reasons that most of us bought them in the first place.
I like to think that when it comes to these boats, we just happen to know better than the rest of them.