Zonal marking is what has revived Bonucci. He was never the type of CB to check his shoulder and mark a striker. He always wanted to anticipate. And that's his best attribute.
So if you play in a man on man system, you are responsible for your own man and your teammates are not looking at the threat. It's all up to you. Mark him or it's a goal.
But with zonal, everyone is anticipating the same threats.They are all watching the ball. Bonucci, Alex Sandro and Matuidi all saw the same ball coming in. So we had 3 options to stop it instead of just Bonucci. But they all failed.
Now, I think some of that is due to playing with only 3 CMs in front of the defence instead of a more standard 5 midfielders. And I said this a few weeks ago. The backpost is overloaded and the fullback is isolated. Alex Sandro sees the threat to Bonucci but he can't commit as much as he wants to help him because he has to worry about his own situation. Because he's somewhat uncovered himself.
And you watch the ball come in, the player has too much time to aim it. And another big problem with zonal is that if the opposition sets up and aims a perfect ball, you're fucked. Because the opposition knows you're zonal, they go into the softer areas and look for a perfect ball. The best example is actually SMS's goal.
And we don't have any wide players, so the opposition will always have time to aim.