The reason is rising of the number of the minimum required champions for Ranked to 20 if they allow 2x5 bans.
That will make new players stuck on level 30 for months.
So we need to find a solution for that, here are some thoughts and the problems that this solutions can bring:
Giving an IP boost pre 30 is so new accounts can get faster to 20 champs with a price of 3150IP is ok, but is bad for riot business and it encourages smurf accounts which is the biggest problem for NEW players to LoL.
The accounts level cap can be raised to 50, but then masteries and runes must be deleted and reworked from scratch; example masteries raised to 50 points or reduced to 25 - 1 point per 2x levels.
There can be a new mode... And the both 2x3 bans and 2x5 bans modes splitting summoners from competing with each-other for material reasons is no good. The only way this could work is if there is a creation of a Junior league(which i imagine is very complex) in which only players that own between 16 and 20 can play, also transferring between the two is impossible, one starts in the Senior from zero.
The** fastest solution** is:
Raising free rotation number of champions to 20(rotation can cycle on 2 weeks instead of one) and including them in the rooster while still keeping the requirement of 16(or 15) champs to play ranked. That way Riot somewhat keeps the initiation for summoners to by new champion and 2x5 ban mode can be enabled.