It's the way Neilsen ratings worked then - if someone had 'non-normal' watching habits, eg. nerds who enjoy watching scifi and fantasy, they would be removed from the Nielsen ratings program as 'outliers'. As this happened all the time, even if scifi watchers increased, they would be culled from the ratings and therefore pretty much any scifi that wasn't watched by Joe 6pack got canned because apparently no one was watching it.
Now that we have Netflix, we _know_ exactly who is watching what, not just a (bad) statistical approximation. And hence we now have _lots_ of scifi and superhero movies and TV because they've not thrown away the data from all the people who would prefer to watch something other than reality TV.