Probably same as Aveline (who I also wanted to romance
). She might be already married, or not looking for romance, or the circle explanation as above, which is probably the most well supported as of now. Remember, our other mages in the DA series were: The Hero of Ferelden (recruited to the GW, thus no longer beholden to circle rules), Morrigan (an apostate, same as the warden), Wynne (a circle mage, not romanceable), Anders (an apostate, romanceable), Merrill (Dalish elf + apostate, romanceable), Bethany (circle mage, not romancable... of course that probably also has something to do with her being your sister), am I forgetting anyone? We've never had a possible romance with a circle mage.