To be honest, the attack damage carry is called that for a reason. There's a reason its not Support Carry. See, even if you do well and manage to feed a dumpster fire ADC without taking the kills, he's still a dumpster fire and will lose the game later on by making it a 4v5 with stupid plays, stupid calls, or stupid chasing.
Also, KDA doesn't have meaning. Objectives have meaning. The enemy team is just in the way of those objectives. That's why you can get your ADC to 8 kills no deaths and then he goes on to chase another 3 kills through the jungle while the rest of the enemy team destroys all of his turrets and actually wins the game. This is the main reason why support can't carry, you can lead an idiot to victory, but you can't make him actually win.
Prime example, I'm lulu and I clearly managed to secure 3 kills, accidentally stole one or two, but it leads to an ace, and instead of going after even ONE tower, my team goes 'LETS GO GET THE OCEAN DRAKE." we're down about 4 towers at that point, there's a minion wave not too far from mid turret, but instead my team decides to get the Ocean Drake. No matter how well you do, when your team decides to avoid the real objectives in the game it's impossible for you to carry.