In my opinion, this is because when people are auto-filled, they are placed in a role that they are less familiar / comfortable / skilled in, but still in a division that is based on the rank they have obtained from their main role.
A Diamond Mid laner is not likely to be a Diamond Support.
If instead we were given a rank by role, when we get auto-filled, we should get placed in a division that is more suitable for our skill in that role.
This would also encourage players to improve their skills in multiple roles because someone in the situation I mentioned previously is not likely to try and queue up as a support in order to improve, knowing they are going to be placed against opponents with a significantly higher level.
I heard rank-by-role was supposed to be coming next season, ... but I haven't heard any more about it recently.