HND-ICN is VERY VERY VERY popular for Korean tourists - they really enjoy the "no sleep, travel all day, maximize vacation" life. That's why there are so many red eyes to SEA - they'll land at 5 AM and go right to work.
There are LOTS of 2a horrible departures to/from ICN for this tourism "cheap" ticket demand. BUT honestly it's not that cheap! People just THINK it is