Because it's BORING to have the same stuff SEVENTEEN times.

(17 because there're gonna be 17 FFs... I know they have not released seventeen yet, but imagine every FF had the same mid-age-bla-bla atmosphere, that'd be horrible.)

I'm opposed to the old flavor (which doesn't mean I don't like the old FF games), but a NEW, non-futuristic flavor would improve the series indeed.