I think it's because the Omega one was always clearly a form of clothing.
The Sin Eater isn't really wearing any clothes. If anything would be considered clothes on said Sin Eater it would be the gold thingies.
If I would take a guess in how SE would implement a Sin Eater outfit it would be in the form of a polymorph.
Kinda like the Succubus transformation in PoTD.
But with the restriction that you can't do anything while polymorphed or the effect will wear off.
But a monster polymorph would be a neat thing to add for Halloween events.
But as actual costume pieces, I highly doubt it.
