Dandadan hit because it's exactly what people need right now: it's got the right pacing, insane visual effects that work perfectly in short clips, and most importantly, it doesn't take itself too seriously. The premise is absurd (aliens vs. spirits), the characters are chaotic and fun, and the action scenes are perfect for 15-30 second TikToks. When you see a snippet of a completely wild battle or a funny moment, you're obviously gonna share it.
Another thing is that the main duo has chemistry that people find genuinely engaging. It's not the usual tiring love triangle, there are jokes that land, and the tone mixes horror, comedy, and action in a way that doesn't get boring. The community started making fan art, compilations of funny moments, theories about how the story continues - all ready-made content that feeds itself on social media.
Then don't underestimate the "it's new and everyone's talking about it" factor: on TikTok and Instagram virality is self-reinforcing. Once a few big accounts start posting content about something, the algorithms push it to tons of people, and magically it becomes "the moment." Dandadan hit the right place at the right time, with a visual style that works in bite-sized pieces and a story weird enough to make you want to find out what happens next.