Why wrap lines in plain emails?
Because of RFC2822 (Internet Message Format), section 2.1.1 (Line Length Limits):


  There are two limits that this standard places on the number of
     characters in a line. Each line of characters MUST be no more than
     998 characters, and SHOULD be no more than 78 characters, excluding
     the CRLF.


Why they chose 77 and not 78 for Drupal, I don't know, but I'd bet my house the decision was originally based on that standard.