There's an obvious difference in status between Pirlo and Buffon. One is Juve's legend, leader and indispensable part of the team, while Pirlo, although an excellent player still, isn't perceived in the same way by neither mgt or the fans; and Pirlo's current place in the team is far from secure, while Buffon will be undisputed starter for a few years at least, thus releasing Pirlo would be far more easy than releasing Buffon.