Why did the Cowboys defense not do good even though they had talent?

They had one of the worst defenses in the league even though basically their whole defense have been Pro Bowlers in the past 3 years. They have lots of big name defensive players with the exceptions of S Alan Ball and DE's Marcus Spears and Igor Olshansky. Why did they do so bad last year then? I know the whole secondary had an off year and the D-Line and Spencer but was it because of that or did they just not execute?