He prefers to have more freedom to step out of defence and win the ball early and start attacks.
The problem is that Mascherano can only play that role and Bartra appears to be that type of player as well.
For the past two seasons Pique has had for a lot of it had to play the more defender/leader role due to Puyols injuries and general decline.
Put Pique back in a defence where he is beside a real leader type type and allow him to play his preferred role and he will be back to one of the best CBs in the world.