That's because we have never beaten the boss, and never will unless they change something for this boss. A lot can't even beat an underling by themselves, a lot don't have dot spells and no one has a dragon or skellies; so how are level 9 - 15s going to do enough damage to the boss to kill him.

PS I've been saying this since the incident started.

As already answered it's due to boss not being killed during the incident, if you want to know next incidents starting time - just look when next incident of other group for same single/two-stage incident will start.