That probably has a lot to do with the amount of kit they get. The top spec diesel dualogic skydome-equipped Y has more kit than a top-spec Audi A1 without options, has more room inside than pretty much all other superminis (longer wheelbase than the Panda so rear seats are actually usable for extended journeys) and costs less than a midrange Punto. Ironically, the same can't be said of the Fiat Bravo/Lancia Delta twins, as the Delta costs a lot more for the same spec, and is more of a status symbol. The Y, and even the really odd one that used to be based on the Fiat Idea have always outnumbered their Fiat counterparts in Italy though, as they've basically been equipped to sell well, while in other markets they priced them into Audi territory in other markets.
Edit: Also, on a more cultural note, as much as the Italians love their tiny little cars that can scream down their narrow cobblestone streets, they're also extremely fashion conscious. The 500 was a chic little retro fashionmobile when new. Now? They probably find it old hat and soooo last-year.