Because DA was terrible and most any other team would have fired him last year. He was lucky to get the third year. But it's not just DA, Payton went through 3 bad seasons and a suspension by the NFL but they stuck with him. And, before that, they stuck with Haslett longer than they should have and even after he created a scandal. Even Mora and Ditka got long leashes.
And frankly, many fans, including me, wanted DA fired after his first year. But they stuck with him for 1 1/2 more years. Given DA's overall coaching record and lack of success he was given more time than was really reasonable.