i think there are multiple reasons
-a lot of americans don't even get to work daytime 9-5 hours and be home for primetime.
-alot of broadcast stations went "reality" show crazy or those dancing with the stars or gordon ramsey type of shows crazy. And people abandoned broadcast television all together.
-related to the previous one, for some reason the sitcoms they do produce get cancelled too quickly and they'll just pick up some stupid baking reality show or a fukking lego show or something.
-broadcast tv should be free on our phones at this point.
-cable shows like Boardwalk Empire, Sopranos, Breaking Bad, Walking Dead, etc...took over because people still care about fictional stories.