It's be cause they don't revisit old adventures. They added armor to bosses because people could use Alexstrasza and win easily, now they printed Platebreaker and forgot to ban it in adventures. That's how I beat lich king with almost every class. The weapon is immune to weapon destruction and is not supposed to be stolen. You need to focus on his minions, not on the weapon.
It's be cause they don't revisit old adventures. They added armor to bosses because people could use Alexstrasza and win easily, now they printed Platebreaker and forgot to ban it in adventures. That's how I beat lich king with almost every class. The weapon is immune to weapon destruction and is not supposed to be stolen. You need to focus on his minions, not on the weapon.