alt Hacker NewsMails 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.
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).