The end of the Primordial War

Once the Primordials were no longer on the Prime Plane, and cast back into the Elemental Planes, Pentharian made the Prime Decree, which marked the end of the Dark Celestial Era, and the beginning of the Divine Era.

After several centuries, Pentharion looked at the balance of the Prime Plane, and decided that it served the true balance now of the universe, and should not be weighted in any direction. While Divine can still have influence in the world, their direct involvement in the affairs' of the Prime plane would be frowned upon.
Pentharion, set forth the Prime Decree, which outlined what circumstances would warrant what action on the part of Divine. It is thus through Celestial Beings that Divine would often work through, though often to avoid pushing the boundaries of the Prime Decree, they would influence the minds and actions of mortals.
Pentharion made this ruling to free mortals, as he feared that if the Divine lived among the Mortals as normal, the Prime plane would become another adaptation of a Celestial Plane, loosing the beauty of the balanced Prime Plane.