Gratuitous Threadjack:
[sblock]If you knew this cat you'd realize how funny it was. I swear the thing is somehow related to a Great Old One, either that or she's the High-Priest of their cult.
I picked her up from a box that had been thrown out of a moving car on the interstate, all the other kittens were dead from the impact. She's blind as a cave-fish about six pounds soaking wet and bright orange. And she can make sounds when she gets angry that I swear I could tape and use as proof of demonic possession. The first time I turned the router on with her in the garage rubbing against my legs she mauled me so bad I needed 37 stitches. It was like I'd stuck my foot into a blender. But otherwise she's the sweetest little kitten in the world if she's not startled.[/sblock]
The Following is purely a purely IMO non-canon take on this period.
More seriously I'd focus on the differences between what happened on the Prime and the events of the Planes.
On the Planes none of the outer planes yet existed in anything resembling their present forms. None of the figures, gods, outsiders, etc, that the current D&D timeline knows at all are present and the planes themselves are largely unformed in a primal conflict of opposing planes themselves. All the things belonging to even the past of the D&D cosmology happen after the primeval chaos goes away.
On the prime things are barely formed. Infant suns lighting worlds that have just developed primitive life, a Devonian or Carboniferous universe just beginning to develop between Outer and Inner Planes which are themselves still forming. As a newborn multiverse just boiling out of the underlying strata of Creation its borders are still weak and malleable. This is how the Great Elder Evils slip in from another multiverse(The Far Realms). they do what they do for reasons that literally can't be understood because their very being is incompatible with this new multiverse they're in. And while they aren't initially troubled their very being is just wrong for this multiverse.
The age of primordial chaos and evil ends not as a result of direct actions of entities but like the entire multiverse having an immune reaction that drives these powerful and malignant infections from it. But the entries they used can't totally heal, so the possibility of return will always exist.