That was done in response to the fallout from 9/11, where it was demonstrated that there was very little inter-agency cooperation, especially in sharing information about terrorism/terrorists.
It was a calculated decision to greatly increase information sharing between government agencies (including the military) to allow for better cooperation.
Well... it seems they did not calculate accurately the permission controls to prevent people like Manning making and distributing a copy of all those files.
It was a calculated decision to greatly increase information sharing between government agencies (including the military) to allow for better cooperation.