Actual answer, as posted by Blizzard years ago:
There used to be no delay at all. But when they added the determination buff in Throne of Thunder, they also started delaying the wing releases.
It’s to mimic a sense of progression. For raid teams, clearing the raid takes time. A raid is supposed to last weeks or months, not be cleared in a single day.
When they added determination, they made it so everybody was guaranteed to clear the entire thing within an hour or two. So the same patch they added determination, they also started delaying the wing rollouts.
Basically, since the fights themselves no longer take time to work through, Blizzard must fall back on using the calendar to do that, instead.
This all started with the Determination buff. You can’t separate the two.