The shorter barrels started getting more popular due to CAS. 20" .357 '73's were all the rage for a while, then they started making 18" models which some switched to. But that is only a part of the equation, especially since CAS is shrinking as far as new gun purchasers go. The models for sale from the various importers is mostly customer driven. They order what has been selling and what they have backorders for. Looking historically at inventory and orders from Cimarron, there have been a more or less equal number of 24" barrels ordered versus 20", except for .357's which trend more to the shorter. The prices are the same regardless of barrel length, so for used 24" to be higher priced than 20" is just an anomaly, IMO. Because you happen to be looking for something that seems to be harder to find right now is just random variation, I would say. Keep looking! And if you are looking for a new gun, get on the backorder list of as many importers as you can while you continue looking for available deals.