In DS3, Irithyll also lacks the dark sun but that's easily explained by the location being "magical", in the sense that's a moon/illusion themed region since DS1.
The sun also always shines brightly in Archdragon Peak (home of the exiled son of Gwyn). I think it's less to do with magical illusions, and more with the power of the local "god" maintaining these celestial lights (or lack thereof).
My impression was was that the story was that the world has been through so many cycles that time and space is breaking apart and getting pulled together.
Locations that logically should be far apart geographically are now close to each other. Things that should have happened far apart in time are now right beside each other.
Everything is converging to a single point as both exemplified by the design of the final area and the lord of cinder. When you're moving through the world you're not just moving spatially but also through time. The most extreme example of this are the final teleports both in the main game and the ringed city.
Essentially there is a reverse big bang going on, possibly before the universe is
really reborn or real non-existence, not just darkness.