I think part of the issue with some of the 'rarer' species of thamnophis is that people who are interested in only breeding a litter or two and selling them off are going to want either something very eye-catching (like a CA red-sided) or something with a ton of pre-existing morphs that they can potentially buy and breed with their snakes (like easterns and radix). Another part of that is that they can be more or less certain that they'll be able to sell the offspring if they breed them. Someone with a large collection would probably be able to keep and care for a litter that didn't sell well than someone with just two or three snakes.
idk, I guess what I'm trying to say is that small time hobbyists that don't have a lot of connections in the herp community and don't have the means to care for large numbers of snakes long term are probably going to be wary of breeding the less common species because of the uncertainty involved with selling the offspring (since they can't look up a bunch of ads and see how much they cost and how quickly they sold like they can with some of the more common ones).