So we shouldn't add important information just because some people don't know what it means?  Wouldn't it be simpler to explain what it means and why it's important to their favorite TV show - since it is the most important factor to whether a show stays on the air or not (demo ratings indicate what portion of adults 18-49, the segment of the population most desired by advertisers, watch a particular episode.  The higher the number, the more a network can charge for advertising, ergo the more successful a show is.  There, that wasn't too tough, was it?)?  
Reporting irrelevant information (like total viewers  many shows with large viewerships get cancelled if their demo ratings are too low, i.e. below a 2) just because it's easy to understand doesn't seem like a valid reason for listing some information and not others.  Ratings are reported by every major news media, even US Today reports the 18-49 rating, so obviously many do know what it means.   Not including the demo 18-49 ratings in an entry about a TV show is like listing baseball games without the score.
As for where to put it, at one point the demo rating shared a box with total viewership in the episode listing  either put it back there or create a new box in the episode listing grid for the demo rating.  Doesn't seem like a big deal to add it.