A big part of the issue is that the nature of the conversations has changed. Mail and Telephones were never at any point perfectly private. The idea of having complete privacy in such conversations is actually rather new.
The difference is that those communication mediums now represent nearly all communication, rather than a small fraction of it, and that the effort to meaningfully break that privacy has dropped significantly over what it would have required to surveil millions of people in the 1950s. It doesn't require an East-German-esque security state anymore.