The Mage vs ADC circle jerk
I will write this post from the perspective of a gold + bellow player. But still this means 90% of the player base. 90 freaking percent.
A bunch of mages are super strong and mana on them is a joke. A good mage will out scale the bot on CS and inflict a great pressure when paired with either top or jungler on an lane.
Mages like Maltz, Brand, Veigar, Cassio or others are a pain to gank. I would prefer ganking 2 Yasuos than a Brand. All these mages are better picks than any assasin and they can protect the ADC better because of their CC. They also provide tons of damage making them easier target for peel. You win lane and go bot and put pressure on that lane like neither non mage top, jungler, mid can do.
Are there mages that struggle? Yes. But that's another topic and riot needs to buff likes of Syndra, Liss, etc
Another bonus is defensive itemization. If I'm a tanky diver, fighter, etc I need to build armor because every game has an ADC. Building MR takes up an item slot and against a Brand for example, is mandatory else I die in 1 freaking R.
Is the game centered on bot lane? Yes, this needs to be tweaked, but this is another story.