Actually it's not explained as of volume 19 of the light novel. But there are some theories around: 1. He simply didn't want to revive because his wife died and he doesn't want to live without her 2. Rimuru is the reincarnation of Veldanava 3. After Milim's birth he lost most of his power and that's why his rebirth takes longer 4. He and his wife are alive and are chilling somewhere (unlikely imo) 5. He lost his authority as "god" because he chose loved his wife more than his other creations and because of that he is dead These are some theories around and it's not certain which of them is true (or maybe something else). I hope i could help you a little