I think there are a couple of separate issues here.
Easterns are generally not as pretty as other milks, and some are finicky. I think that explains their lack of following.
Scarlet Kings are too small, high strung, and babies will only eat skinks. Even some adults are tough to switch off skinks.
Red Milks are gorgeous and make pretty good captives. The babies can be tough to start, but they aren't any tougher than many other species of popular Lampropeltis. However, red milks are a dime dozen in certain areas. People can go out and catch their own if they don't mind flipping a few rocks. I think that ready availability of snakes makes people less inclined to breed them since a lot of folks would rather catch their own. I don't know this to be fact, it just seems the most logical explanation for their lack of popularity in the trade. I had a WC red milk for several years and it was a great captive.
I think the western ssp suffer from the fact that some populations are drab and the really gorgeous populations aren't easily accessible to the hobby.
If you aren't a milksnake purist, why would someone pay top dollar for a cb Utah Milksnake when they could buy 5 cb annulata babies for the same price?
Just my thoughts,
Chris
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Chris Harrison
San Antonio, Texas