I don't know the exact reason, but I know two reasons which would explain such setting.
In a console, a line can only contains 80 characters. Some people use mail clients in a console.
Another possible reason is that long lines are difficult to read (AFAIK, that is the reason why LaTeX uses wide margins, and the reason why text in newspapers is devided in columns.
The returns in an email are important because when you reply to a mail, a ">" is added in front of every line.