Gideon Demoralise on Raid Bosses

Is there any reason why demoralise on Gideon's kit doesn't do anything at all on raid bosses like other unique buffs/debuffs do, i.e. fracture reducing raid boss speed by 50%. Having something on his kit applicable to raids along with him being able to apply armour shred would make him a lot more viable in raids, whereas right now he's next to useless as a raid character