It said it in the good book and don't forget no death goes without the Lich King's say since he is the true god of death. So when Sargeras killed the Pantheon, it was because the Lich King allowed their deaths to happen. Except in this case the ant won't die no matter how many times the T Rex stomps on it while the ant can kill the T Rex after just one bite.
Sargeras is stronger than Kil'jaeden, we all know that but what does that have to do with the Lich King being weaker than Sargeras? Kil'jaeden created a monster that went infinitely beyond what he and Sargeras could ever hope to imagine and now they both fear the Lich King. Sargeras cannot strip his powers, its more like the other way around where the Lich King can either enthrall him and turn him into a undead slave or one shot soul kill him with Frostmourne, or swarm him with infinite undead dragons, take your pick.