Efforts to end-to-end encrypt e-mail have been disastrous, coming down to a combination of human factors and difficulty of coordination - but mostly, people want to be able to read their mail. Sometimes they want to read it from public terminals. Sometimes they lose their phone and still need it to be accessible. Often, e-mails are required to be unencrypted by the mail server for compliance purposes - Nearly all financial data has to be archived, and that's often the crown jewels you're trying to encrypt, anyway.

I don't know of a good oral history of PGP, but I suspect if you find one, it'll have the answers that you're looking for.