Um, even Cailan doesn't think it's a real Blight, though. Duncan and the Grey Wardens know because of the archdemon appearing in their dreams, but for a man who already distrusts Wardens, with a pretty pragmatic attitude, it would be difficult to convince him. Saying, "Oh, I saw the archdemon in my dreams," wouldn't exactly help the cause. Compared to such nebulous knowledge, it isn't inconceivable for Loghain to determine Orlais is the greater threat.