Originally Posted by
Meiffert
There is something called progression in this game.
You pull a boss multiple times, wiping each time, but learning how to counter the fight mechanics over time.
Even strong mythic guilds, who tested the fight on PTR, are using some sort of voice communication and have very good coordination wipe multiple times on the end bosses on heroic (such as Archimonde, Helya or Gul'dan) the first time around.
For a somewhat weaker group of heroic raiders, the final boss like Archimonde or Gul'dan will likely require multiple nights of progression to learn the fight well enough to down him.
How do you want to achieve that in a pug? Even if people weren't leaving after 1 or 2 wipes, the group will inevitably disband at the end of the evening. Then you build a new group tomorrow and... do what? Start over?
I'm not saying it is absolutely impossible to progress in a pug, but it isn't something many people are interested in doing.
On top of that, for people who have already learned and killed the boss multiple times, it would be incredibly tedious to spend many hours every week to progress again.
It makes much more sense to get a group of people who have already done the work and learned the fight. And the easiest way to confirm this knowledge is having the kill and the related achievement.