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.
No, club sponsors and player sponsors are two different stories. Del Piero's sponsor is Adidas, but his club sponsor is Nike. Even moreover, his NT sponsor is Puma. Or Ronaldo's sponsor is Nike, but his club sponsor is Adidas. But players whos clubs or national team's sponsors are the same as their own sponsors have an advantage simply because then they can wear their own club/nt jerseys in the ads
Thats why in Road to Lisbon, you saw Raul, Zidane, Viera, Ballack and Kahn wear their NT jerseys because they were made by Adidas. But players like Makaay (Dutch jersey is Nike), Del Piero (Italian is Puma), and Beckham (Umbro??) just wore general Adidas sporting wear. Not a big deal, but it would have been cool to see DP in the Italian jersey, rather than some grey tracksuite with three lines on the side...
That is why in the picture in this thread, Zambrotta who is sponsored by Nike, was allowed to wear the Juve jersey because Juve are also sponsored by Nike.