Very simple. On Civ 6, I struggled in immortal. Diety was impossible. This gave a level of purpose to getting better. Deity required either real skill, or a combination of luck and overpowered civ. I usually relied on the later, and still generally lost.
I've played about 10 Deity games so far, with no save scum, and have yet to lose a single one. This greatly decreases the reliability of the game, as I feel like I know i'm going to win from the start.