The Dallas Cowboys have a very strong fan base because fans that experienced the glory years of the 70's are now in their late 40's to late 50's. This is the age that most Americans have their highest income during their working years. Therefore, the Cowboys have fans with a very strong buying power.
The 70's, reinforced by great success in the early 90's created fans that have been loyal for so long that so many aren't going to stop despite the mediocre results over the past 20 years.
Lol, in a way, we are a little like heroin addicts. We have experienced the euphoria of watching a dominant Cowboy team for an extended period of time. We are hooked. Now our dealer has us where he wants us. He can sell us less potent heroin and we will complain, but we can't stop going back to him for more, no matter how he much he dillutes our drug. We crave that same rush of the good days and we'll even believe him when he swears that, this time, our "stuff" is pure.