To get 1st weekly in Youtube Korea. If they released at the same time Melon ULs would be higher, but she wouldn't have got 1st weekly in Youtube Korea which she just barely got, and potentially no PAKs.
If IU truly cared about debuting at 1 on youtube weekly then she would've released the song on Monday like most kpop groups, instead of Wednesday.
But I do think initial MV hype massively helped her youtube streams, and IU wanted people to focus on the MV as a medium to enjoy the song vs simply digital streaming.