There could be many reasons, ranging from greedy ownership to poor drafting to insufficient management to the coaches. In the case of this particular franchise, probably all of the above and more. Having never followed the Cardinals on a regular basis, I don't know for sure.
I do, however, know one thing: They finally made the right decision when they hired Ken Whisenhunt.
Whisenhunt played professionally as a tight end between 1985 and 1993 with three teams, then went on to coach various football programs at that position. I personally first heard of him when he joined the Steelers in 2001.
Now, I'm not one to follow tight ends on a regular basis. Occasionally one jumps out at me for his abilities as a receiver, or substantial blocking ability, or just because he was drafted in the first round. Whisenhunt did a fairly good job in Pittsburgh on most accounts, but it wasn't until he became offensive coordinator that I started paying attention.
With Whisenhunt leading the offense, he quickly became affectionately known as "The Wizard" for his ability to balance the Pittsburgh running game with well-timed trick plays.
Immediately after winning a Super Bowl in 2005, he was offered the chance to interview for the Oakland Raiders job. According to accounts I read, he was offered the job but turned it down, due to the extremely limited control he would have on Oakland's football management.
So, back he went to the Steelers for another season. And, finally, he was offered the chance at the Arizona position.
All in all, it's been a fabulous ride for Ken, and it's great to see the Cardinals franchise made a terrific decision to get themselves on the radar.