If I may hazard a guess, I'd say it's because they are both pretty culturally and historically significant cities. Both music capitals (Memphis for blues/rock and Nashville for country), both home to famous figures (hence the Elvis and Andrew Jackson questions), both major civil rights battlegrounds--plus, Memphis has the distinction of being one of the "blackest" major cities in the US by percentage of its population. I've never been to either, but as a music/history nerd, I'd definitely like to visit both at some point.
I remember Silence Of The Lambs being in the FBI headquarters in Virginia, Lechter in Baltimore, the murder sites in Ohio and West Virginia, and a decoy house in Chicago. Where did Memphis come in?
A great one, and not difficult for one who has lived in Nashville in 2005 and 2005, plus visited Nashville on business trips for about 15 years' time. The movie question had to be checked, and another guessed. Then 100%:))
After moving from Houston, but before changing their nickname to "Titans", the NFL's Tennessee Oilers played their home games in Memphis in 1997 and Nashville in 1998.
I'm skeptical that "Athens of the South" is used with any frequency. I've never heard it called that. And it's a bad nickname because there IS an Athens of the South: Athens, Georgia, where the University of Georgia is.
I read it in tourist guides before visiting and on internet searches, has a lot to do with the number of universities. Whether anyone who lives in Nashville ever uses it, unless they work for the tourist board I cannot say
Have a good day !