There are a few reasons why Juve got that 2nd goal. The smallest might be Buffon´s save. I mean he was part of the action, so he could have done otherwise, but i wouldn´t say it was a mistake or he could do better. It was bad luck that the ball bounced that way. Messi shot hard from the line of the box, Buffon is standing close to the 6y box, so he is about 12y away and the ball bounces right in front of him. He himself might also take the blame himself, because he wouldn´t throw his teammates under the bus of course (and with a little luck, the ball bounces 20y away).
The first problem was the poor defending of #15 Bonucci, who lets Messi dribble past him 20y away from the goal at full speed? That is asking for a goal. He has to stop him, either get the ball or foul him and let them have a free kick.
Equally minimal bad but part of the goal is Evra, who is just looking for the ball, but i think so close to the goal, that just happens. I guess in a more lucky situation he ends up with the ball.
Another poor showing is Pirlo. This just perfectly shows his tempo. He might run 15km, but he jogs. Look at 2:15 of that video, when the play starts and Rakitic has the ball. Pirlo is 5+yards closer to Juve´s goal. He jogs. Rakitic sprints and then it´s a 2-on-1 where Rakitic+Suarez are vs. Evra and Pirlo is trailing them by 5+yards. He could have sprinted and he could have had that ball, if he would still be 5+yards ahead of Rakitic. I know he probably never does that (sprint back in own box).