I think he simply didnt feel like it.
Even if he sucessfully defended his title for one year, he still has to hold it again and again and it simply seems like too much of a hassle.
Magnus also seems to be taking the "Content creator" path rather than just professional chess player.
Magnus has always won World Chess Championships by a tiny difference, but whenever Nepo gets nervous he blunders like a 200 rating player.
Was Magnus afraid that Nepo learned to control his nerves for this WCC? If not, why Magnus rejected defending his tittle for WCC 2023??