I don't think I'll ever be able to wrap my mind around the way TV execs think. I know the main 18-49 demo is important for network shows, but you would think they would just be happy that people are watching their shows.
But their thinking is that a different show in the same time slot might be better at drawing the demographic they want. For them it's not just about an individual show, it's about whether that show is contributing enough to the overall schedule, and whether they think a different show could do better.
Although it does have a lot to do with how they define the goals of the network overall. Unfortunately, the CN executives apparently believe that goal is to sell toys to young boys as efficiently as possible, rather than having broad appeal and critical acclaim. Granted, I've heard a lot of critical praise of some of their shows like Adventure Time
(though I could never get into that), so there's still room for good shows, but only if they're aimed at young male viewers. Which is why CN has become such a hostile environment for shows that we older types enjoy.