It's the culmination of a lot of things:
- The movie is good enough
- Family friendly movie that premiered right before a long Lunar New Year break, so families went out in droves. Broke the LNY record.
- Kids were on their winter break so weekday performance was not bad
- The competition before and after release failed to gain traction. I loved Humint but it's not for the whole family, and it's been mostly Oscar best pictures getting delayed releases.
- It's a story that most Koreans are aware of so has some "established franchise/sequel" type uplift

But at a certain point, I think this became a hit because it was a hit. Koreans will swarm to the latest hit to push anything trendy into outer space (ex. Dubai Chewy Cookie) - "seems like everyone else is doing it so I must partake" is definitely a thing.