The single gene morphs that have been around for a while aren't in as much demand simply because everyone has one already. Most of us have them mixed with another morph. For example, I don't have any spiders, but I have a bumblebee and a spinner. I have one straight pastel, but he's not a good example of pastel and browned out quickly, so he's a pet not a breeder. I have pastel in plenty of other critters anyway.

The breeder market on the morphs that have been around for 10+ years is pretty saturated, which is why they don't cost much more than the normal BP's.