Pretty interesting article about the failure of PKIs at Trust Cannot be Outsourced (via Financial Cryptography).
Another major reason why PKIs failed is the fact that vendors claimed they could solve any problem at all. Companies started to implement PKIs without any good idea what they wanted to accomplish with them. Thus, the projects often became bloatware and died a quiet death. Or, they were successfully implemented but there were no applications making use of them. Quiet death once again. \via [Axel_]_