Because Vieri was a good player back then.
Vieri these days couldn't hit a cows a.r.s.e with a banjo. ;-)
He is infected with a virus I like to call Perdentitis, which causes players to miss penalties, tap-ins, scudetto's and makes defenders go for the player instead of the ball (Materazzi).
The time that this infection takes place on players during their stay at Appiano Gentile varies. It could take years, it could take months.
As we have seen so far this season, Vieri has well and truly been cursed with the disease now after 4 years in La Pinetina. Adriano is only a newcomer so it is very rare for a player to be infected so early in his career at Perdenti FC.
The cure is well known to supporters and players alike, but undertaking it takes one big dose of courage. Turning your back on Massimo and joining Juventus in Turin (Cannavaro) or showering in success across the city via Turati (Seedorf, Pirlo) are the two quickest cure of this contagious syndrome.
As it is, very few players are immune to one of the most contagious diseases in the football world so players able to perform in La Pinetina are few and far between. Currently the only known players with immunities to Perdentitis is their captain Javier Zanetti and newcomer Adriano.