They are short lived, about three to four years, with three probably being the average, makes it kind of hard, you would have to maintain a colony to be productive over the long term. A second factor is that the wild caught are very cheap, that keeps many species from being captive bred regularily. Also, with most captive breeding there is an insentive for the buyer, usually captive bred are better adjusted and have better temperments. In the case of green snakes, the babies are small and relatively delicate compared to the wild caught adults that are available, and generally their temperment is just as good either way. Other then those economical factors, there is nothing preventing successful breeding, many people who post in this forum have done so.