This is not a noob question, please read.
So I'm rewatching the old Warcraft 3 in-game cutscenes again and something really kind of caught my attention this time. Following the fall of Dalaran, and after Kel'Thuzad summons Archimonde, Archimonde immediately designates the dreadlords the new rulers of the undead.
So my question is, what was the Lich King doing up to that point that the dreadlords would not have been capable of doing. And if the dreadlords were incapable of managing the scourge before Archimonde arrived, why not just let the Lich King remain in control of the undead thereafter, considering how good he'd been at it up to that point.
What's the point of sending the Lich King to Azeroth in the first place if the dreadlords could have run the scourge and spread the plague the entire time anyway?