It's because universal truths and themes are easy for all people to identify with, and enjoy. Its a good move on SE's part because it somewhat links the games together in an abstract way.

Hell, look at even the main character names from the past games. Cloud, Squall, Tid(e)us, Lightning... all can be linked back to simple things like weather.

I'd say the element of uniqueness could also be included. Maybe not to the storyline perse, but more to the style of play of each game, how each FF is different from the last, hence the "Final" Fantasy.