Well, if you check Adidas's website and go to the Road to Lisbon section, you will see a nice big wallpaper of Del Piero there.
See DP isn't sponsored by Nike, therefore, why would he be in Nike commercials? In addition, since Juve's jersey is made my Nike, Adidas would not allow DP to wear the Juve jersey in their ads. And just for the record, DP was Italy's only representative for Adidas in the Road to Lisbon ad that we saw continously before, during halftime and after the Euro games. Along with Totti, DP is Italy's most popular commercial icon. Of course the likes of Zidane, Henry, Ronaldinho, Ronaldo, Davids and Beckham get more attention in the ads because of their greater individual popularity, but that doesn't mean that they are on better form than the players who get less commercialized.