Supply and demand. They are available over a smaller natural range than many other animals, small quotas in the wild, they don't do well outside their native range and they are highly desired.
There have been a few discussions on here mentioning problems with breeding sable, from aggressive bulls killing each other to mortality due to cold, drought, etc that has a far greater effect on sable than many other animals. there is a Sable study that I recently read detailing their requirements that really shed some light on the subject, rainfall in particular seems to be a major restricting factor, they only occupy areas that receive rainfall amounts that fall within a narrow range.
Having said that I am sure there is much more to it as well.
