ranked 5s

Is there any logical reason aside from queue times on why ranked 5s are only available at certain times? Many of my friends cannot play until after 10pm and usually we would have last season got on around midnight to play 2-3 games and wind down before bed. I mean yes we could do this in normals but it just seems like a waste of time playing normals now adays. Im just curious if there is solid reasoning for it aside from the inevitable riot wanting dynamic queue to work so they need the queues to be low times.