I think the problem nowadays (and this happened/is happening to The Walking Dead too) is that the shows aren't as good as they were a few seasons ago. However they still outdraw everything else the network can offer and still get the most viewers. So the network has no reason to pull the plug on it. Even if they aren't getting the number of viewers they were at the beginning or even if the show sucks compared to what it used to be, it's still their biggest money maker and they will milk it until it's dry.