It's not the bosses that need to be killed. All the mobs have to be killed too. A few years ago we had like 4/5 groups going around clearing mobs this year. There was probably 2 (who spent most of the time fighting) then some solo players.
As each wave of mobs died the next spawns. There wasn't enough mobs dieing to bring the next spawn. By the time the 3 hours is over only half the waves needed to spawn the boss was killed.
But when Pve players and guilds can't clear the waves without being pked. Ment they stopped taking part.Which ment in the long run all the higher level bosses never spawned and the main boss didn't spawn. Either.
These are community participation events. But the community here would rather fight over a vanquish brace and g oil than progress the event. Plenty of Pve players like xarris who would chop down several hundred mobs solo was pked for lvl 120 bosses. Meaning they stopped doing the siege. But instead of leaving people to get on with clearing the low level spawns. So we could all benefit from the bosses that spawned later it's much better to have a 12 inch e penis for stealing a 120 troll, biting your nose off to spite your face. As the saying goes.