Here's why, according to Eurojust:
Criminals in Italy are increasingly making phone calls over the internet in order to avoid getting caught through mobile phone intercepts. Police officers in Milan say organised crime, arms and drugs traffickers, and prostitution rings are turning to Skype and other systems of VoIP in order to frustrate investigators. Skype's encryption system is a secret which the company refuses to share with the authorities. Investigators have become increasingly reliant on wiretaps in recent years. Customs and tax police in Milan have highlighted the Skype issue. They overheard a suspected cocaine trafficker telling an accomplice to switch to Skype in order to get details of a 2kg drug consignment.