Why do Italian zip codes not work?

New ZIPs - postal codes

In 2006 the Italian postal service has changed and added a number of ZIP codes. As is well known, each Italian town has its own zip code (there are over 15,000 different locations, which are assigned a postal code). There are 27 main cities instead, which are divided into postal zones, and each of them has its own postal code. E.g. the general ZIP of Rome is 00100, but Via Flaminia Nuova has now its own code.