The diary has always been just one week. And it ended on a Thursday which was, long ago (as in "the later 60's") determined to be the best time as it gave people all day Friday to finish filling it in and the weekend to drop it in a mailbox.
Since then, it has just been "how it was always done". I suspect that the vastly different workdays, the change of jobs from manufacturing to services and all kind of other social changes have made this less a truth than a custom.
And, of course, the top 48 markets are measured electronically with participants households remaining on panel as much as 2 years if they fully comply.