Almost purely the variety of random maps (and, on TSL maps, the variety in Civ arrangement and challenges of different starting positions) - the game tails off noticeably once the exploration phase is done. I liked the strategic element of older Civ games, but that's largely lacking in Civ VI and I prefer sandbox games to have a bit more of a sandbox or roleplaying structure, a la Crusader Kings II, that Civ by design isn't really able to accommodate.
Civ V did a reasonably good job on the roleplaying front with its more complex diplomacy and leader personalities, but that was actually one of the less popular aspects of the game in part because the AI was easily exploitable by min-max players and so sadly is a thing of the past. I can't get the satisfaction in Civ VI of seeing a familiar face like Nebuchadnezzar and expecting a certain behaviour pattern, establishing and navigating
relationship triangles, playing one civ off against another until they go to war and I happily open borders to both sides so they can wipe each other out, or the joy of my ally Catherine unexpectedly nuking a Babylonian city after I asked her to join my war.