[layer8]

Mails are (or used to be) processed line-by-line, typically using fixed-length buffers. This avoids dynamic memory allocation and having to write a streaming parser. RFC 821 finally limited the line length to at most 1000 bytes.

Given a mechanism for soft line breaks, breaking already at below 80 characters would increase compatibility with older mail software and be more convenient when listing the raw email in a terminal.

This is also why MIME Base64 typically inserts line breaks after 76 characters.

[SoftTalker]

In early days, many/most people also read their email on terminals (or printers) with 80-column lines, so breaking lines at 72-ish was considered good email etiquette (to allow for later quoting prefix ">" without exceeding 80 characters).

[bjourne]

One of the technical marvels of the day were mail and usenet clients that could properly render quoted text from infinite, never ending flame wars!