Possibly security. When the IBM 370-class mainframe was born, it was often in a data center with competitors running on the same machine. That machine was a "

Possibly security. When the IBM 370-class mainframe was born, it was often in a data center with competitors running on the same machine. That machine was a "universe" and its many software components (various DBs, communications, file structures, language modules, etc.) EACH have its own security. No one can do ANYTHING without explicit tediously-granted authorizations - there is no such thing as a global "root" access. If anyone has EVER hacked the traditional core systems (where the data is), it's a well-kept secret - versus the not uncommon news of "modern" systems being hacked.