with Arhas dead and Bolvor in chage of the undead the chances of them trying to overtake the world a 2nd time is pretty slim, or is it?
think about it, Arthas was the LK, how got his powers for Ner'Zul (spelling?), who got the power from who? Kil'Jaiden, KJ is still alive and kicking somewhere out in the twisting nether with Sargeras, and as we all know one day far far away were gonna kill Sargeras.
But before we can get to him we gota take down KJ, so wouldent it make the most sence if your KJ for when you finally do come to Azeroth to kill us you take command back over one of the STILL strongest forces on the planet?
Bolvor can play jail keeper all he wants, but KJ is teh REAL boss. He has the ability to walk into icecrown, walk up to Bolvor and go "Hey, sup? Yea, im taking the scorge back" /kill Bolvor (just so he cant interfer).
TL;DR KJ is teh REAL lich king and can take his army back whenever he wants and try and take over the world.