The legal precedent come from a time before digital records and generally available encryption so it was largely true that you didn't have a general expectation

The legal precedent come from a time before digital records and generally available encryption so it was largely true that you didn't have a general expectation that papers in the hands of third parties would be private to you, inherently they possessors would have knowledge of what was on those documents. Beyond that it just hasn't really been revisited in depth since the advent of generally available encryption and the internet. Some states have actually passed laws extending the 4th amendment warrant protections to information held by 3rd parties though.