It's hard to track down concrete reasons for 5YL's existence. Especially when many interviews with Levitz, Giffen and the Bierbaums are lost in time in print magazines.
But, from what I've come to suspect, it appears to have been an evolved decision to distance itself a bit from the Levitz run after he stepped down while more thoroughly erasing all Superman and Supergirl references to 'fix' the franchise once and for all.
And then the dystopia setting was added in part because of Watchmen's popularity at the time.
Here's a couple interesting articles I found:
https://notthebeastmaster.typepad.co...ive_the_l.html
http://toobusythinkingboutcomics.blo...fens-five.html
And a reflection on the first issue, by Tom Bierbaum-
https://itsokimasenator.livejournal.com/910.html