Have you seen the chain of events in World of Warcraft? Everything was according to the Lich King's plans. He could have conquered Azeroth without lifting a finger if he really wanted to, as stated by Uther and others. The Scourge had that power. The Lich King just wanted himself an elite, so he let them train themselves against him.
The Lich King would fail to do what the Burning Legion and Sargeras tried to do. Sargeras was never after destroying Azeroth, he wanted the Well of Eternity. Only problem was that he had Deathwing and others to oppose him. Lich King has not had any of the great names against him.
Ysera has overslept, Deathwing has been in hiding, Malygos went insane, Nozdormu does not care and Alexstrasza does not want to fight. Azshara's men as well as Illidan opposed Sargeras. Illidan for one almost destroyed the Lich King twice.
Basically, Sargeras was shit out of luck. The Lich King did not have the same problems he did.