Too many champions for new players
This game is just getting too complex with every new champion. 138 champions is just insane complexity. knowing how the champion you choose interacts with other champions on your team and against you is so hard to know. At minimum you would need to play 138 custom games to lvl 6 to even try all the abilities and that doesn't account for scaling which varies greatly from champion to champion nor does it account for the fact that you would have to own all or nearly all 138.
For instance, if don't know Zyra, then you don't know to crush her plant seeds. If you don't know Rammus, then you don't know how much damage he returns to an ADC that hits him - I've seen numerous ADCs kill themselves hitting him.
Compare that to a classic game that is considered very complex - Chess. Chess has 6 different pieces and each with much less complexity than any single champion in League of Legends. Yet Chess is very very difficult to master.
I will admit that the 5 bans (from 3 before) does help, but only if you know which champions to ban. However, autofill counters those 5 bans very heavily.