How is it that the Djinn made a sword specifically to kill something like Ratash, but never thought to use it at the time?
From Razia's explanation, Solomon and the other Djinns were on the ropes because of Ratash' costantly-expanding army, so they decided to put everything they got into sealing him and the army. After that, most of the surviving Djinns left the area, Solomon and his followers soon died out and Razia was the only Djinn left to guard Rakem and the Sword. Probably they didn't want to open the can just to slay Ratash in case the Army got out of control again.
