Most where you see true 24 hour operations are airports with a lot of long haul connecting traffic. Common at the big airports in the middle east for example (DXB, AUH, DOH, even IST to a point) but no need in North America, Europe, etc where its easier to operate at hours convenient to both origin and destination. I think SIN, KUL, and BKK see a handful of overnight departures as well but in the majority of the world there's just not much need for post 2am and pre 5am.