Advent of Code has this mass hysteria feel about it (in a good sense), probably fueled by the scarcity principle / looking forward to it as December comes closer. In my programming circles, a bunch of people share frustration and joy over the problems, compete in private leaderboards; there are people streaming these problems, YouTubers speedrunning them or solving them in crazy languages like Excel or Factorio... it's a community thing, I think.
If I wanted to start doing something like LeetCode, it feels like I'd be alone in there, though that's likely false and there probably are Discords and forums dedicated to it. But somehow it doesn't have the same appeal as AoC.
The second is the timing and pacing - the fist few days are about warming up, then comes a couple decently challenging puzzles, after which the whole thing gets very difficult. Having the discipline to actually spend the time every day to do the puzzles feels like going back to the gym and actually sticking to it.
I also get to solve these kind of coding puzzles at work very rarely - maybe once every couple of months - so the whole thing feels like an intense workout for my brain.
The downside is of course is that it's exhausting - later puzzles often took 1-2 hours for me to solve - during days where I have work related stress, this is not easy.