I'd attribute that to a combination of Empyrean weapons being relatively easy to put together(and being at the height of their popularity) and your potential sellers having put the Chloris or Ulhuadshi set together predominantly for the trial item. I suppose there's also a small part of the playerbase who still believes the R/EX item <-> gil exchange is the root of all evil, but the likelihood of wanting to buy something from a tardgroup of that caliber is low.
Regardless, Empyrean weapons are only a faction of what can be purchased from a pool of spoils, even if they do make the most impact.