If the bans went to the first 3, then the last 2 picks on each team would be sitting there dominated by the first picks again.
Excuse me, but do you actually remember why first picks were "dominating"?
Why, of course you don't.
It would've been fair for last picks to have bans in Seasons 1 through 5, because the lower was your pick, the lower was your chance to get the role you wanted, due to Pick Order being the rule.
Not in Season 6 and 7 where everybody gets to pick the role they want without fail.
There is zero advantage for being a first pick in Solo Queue, because the possibility of a champion trade is nearly non-existent, while it makes no difference for premades.
Moving the bans to the last people allowed them some power in the pick process instead of being at the mercy of the first few players.
Last picks currently get:
- roles they want
- a very high chance to counterpick the enemy
- an ability to shield their picks with bans.
Meanwhile first picks:
- open themselves to getting outright counterpicked
- don't get any sort of protection for their picks
- are at the mercy of last picks to ban champions they will have problems with - which doesn't happen most of the time, because Yasuo must be banned, even if he's a shit champ that's easily counterable with most champions.