However, Advertisers look more at the number of people 18-49 viewers since they are the big spenders than overall viewers. NCIS had a 3.4 rating this week while the Fringe finale was 1.4 so even then more younger folks like NCIS better. I think the main problems is Fringe is very serial and not everyone likes Science Fiction where NCIS is not serial so you can miss an episode and still not know what's going on and it's very relatable especially given the current events over the past 12 years.
At the end of the day though if the show is not being watched then the advertisers won't want to advertise on that show which in turn means tv channels won't be paid so they really have no choice but to cancel some shows. It sucks but there's not much you can do.