I can tell you, it didn't originate with government. Solicitor-client privilege attaches to corporations, for example, in order for them to conduct their business. It's simply been a necessary doctrine that allows decisions to be made on an informed basis. Government, perhaps even more so than private interests, needs to make its decisions on an informed basis that not only deal with narrow partisan interests but with the public interest as well.
On June 19th, 2006. See this statement in context.