I really don't know reason

i think it's because Jr was trained for like years and years but Aron only trained for months before they debut and he was the last to join nu'est so it is not fair for Jr since he was the oldest before Aron came....

Yah...it's maybe the main reason

maybe because aron is from the states so there might be a lack of understanding in some areas so they went for JR as the leader