The crux of the matter is "knowing exposure to third parties." So you have an expectation of privacy in sealed postal mail, but not say post cards or anything printed on the outside of envelopes. While a postal service as a matter of course can't read what's in peoples' sealed letter mail, e-mail is sent in plain text and can be seen by any intermediary SMTP server as well as system administrators of the sending and target mail servers. That's just the nature of the protocol.
Moreover, the fact that the U.S. Postal Service is an agency of the government puts it under heightened scrutiny as compared to e-mail providers who are private parties. The fact that people would be horrified if the U.S. Postal Service were tearing open envelopes and scanning mail to send targeted catalogs, etc, to people but accept Google, etc, doing the same thing as a matter of course really cuts at the knees of the argument that people have the same "expectation of privacy" in their e-mail as they do their letter mail.