I suspect it's because Pleikki and animal fighter oversupply the market opening Kurzick and Luxon chests looking for shields or cookies or something.

Granted the rarity we perceive of each could be a bit biased based on the skins. I know there are a few Jade/Amber fans out there, but there don't seem to be as many as a lot of the other Cantha-only skins.
Cleav, thanks for the clarification earlier. I think I understand the goal now. Have you considered listing criteria in the OP at all? Some kind of statement explaining that we're talking about max gold items with "good" inherent mod like 20/10 or matched 20/19-20 for skins which can't be found with inscription slots?