Well, technically If Ganon traveled through time to the future using the power of the Sacred Realm (Triforce) then he wouldn't exist in the past, or the time that he was originally from. Thus he was sealde in the Sacred Realm, where the key to opening it is the Master Sword. That explains why Link isn't allowed to touch it again (Zelda told him).It's a very complicated subject, but I hope that explains what you wanted to know.