It is probably because the Cardinals never turn it on until like September. They are mediocre at best until they make a very late push. The days of 100 win seasons were 6 or 7 years ago. Being .500 on August 15th every year isn't all that inspiring lol. I think that a lot of fans expect more from the Organization. But, the Cardinals last 2 WS were on 83 and 90 win seasons, so nobody should give up on them per se.
Another thing is the Cardinals never sign big names to get people excited for the season. They sign guys that usually turn out decent, but they never sign anyone relevant until they are in their mid to late 30's. Which at that point its a crap shoot on what you are going to get. Their roster is a giant question mark heading into march and april every year and that may have something to do with it as well.
Also, pretty much everyone has DVR's now, so I am not exactly sure how that is calculated into the ratings... I never really did any research on that. Just another theory if it is tabulated correctly when people can record it and watch something else simultaneously.