If they added romance, they wouldn't have focused on the plot and this would have made the drama lose its charm. You know if it had romance, the writer will have to focus on the romance and it will take space in the drama. How would you they have shown more case solving and exorcist if they would have added romance. For me, it was perfect being like this. I believe that not every drama has to have romance otherwise it wouldn't be different from other dramas.