When I was growing up (in the 80s) Nick at Nite was all TV shows from the 50s. Why is it now 80s-90s stuff?

Because we are now cled as being old. Sorry to say but it's true. The shows we grew up watching and loved are in now in rerun land and the kids are just discovering these "old" shows. Not that they are that old. Makes you think about what our parents feel when we watch reruns of their "old" shows, huh?