As someone who primarily runs grim dark horror games, D&D, specially 5th ed, isn't really that good. Out of the box, without house rules, it just doesn't mesh well. To put it bluntly, PCs are just to powerful. Lv 5 party can have long or short rest when ever and where ever they want with use of Rope trick and Leomunds tiny hut. There isn't enough sense of danger.
Ravenloft, as a setting, was cool cause every Domain was different thematicly and culturaly. From Tristenora (primitive iron age) to Barovia (medieval) to Richemoult (chivalric).
Curse of Strahd is popular because it's good adventure. Plain and simple.