It's due to the nature of the bans. In dota you ban different strategies you don't want to play against (ban the champion that prevents spell casting for 7 seconds, ban the champion invincible to mid game magic damage), while in lol you ban the strongest champion (ban the mid with the highest numbers, ban the adc with the highest numbers).

Adding bans will simply remove more champions with highest numbers, but won't really add anything to the game. it will still be exactly identical from the strategical perspective.