"I have a great idea! What if everyone could do the story whenever they wanted?"
"That's stupid. Why don't we put an instance at the very beginning, but then give that instance limited slots?"
"But we know everyone is going to be logging on at the same time...so why would we do this?"
"Because like...these instanced fights are always so much fun. This time they get to a slightly different looking magitek thing!"
"Oh, well that changes everything! Why didn't you say so sooner!"