By Diabolos.Sovereign 2011-03-13 19:23:55
Simple answer: Rarity.
It's only got one source, the Campaign Ops BCNM "Plucking Wings", which is highly server dependent. Access to the Ops has multiple restrictions including a Windurstian allegiance, a high Campaign Rank (Iron Emblem of Service or higher), as well as requiring Windurst to have control of Castle Oztroja (S).
With so few people in Campaign these days, the chances of Windurst controlling Castle Oztroja (S) and unlocking access to this Ops is basically nonexistant. Most of the Roundel Earrings available for sale are likely from a time last year when Campaign was dominant, or transferred from a server where they're less expensive.
I know on Diabolos, so many people are allied with San d'Oria that we regularly obtained control of La Vaule and gained access to Witch Sashes and Cuch Mantles... but only a handful of times last year did we gain access to Plucking Wings and Roundel Earrings.
On that same note, Diabolos literally had ONE WEEK where you were able to participate in the Shadow Lord BCNM, and obtain Allied Rings. We've never seen access to that BC again.