He was injured, for one.
He is a "glue guy" that on a talented team goes unnoticed but has a role. Like Heath.
Cheap depth, okay. Beyond that, no bueno.
He's simply not very good. He never become the player he was paid to be. And he was shuffled around and shoehorned from one position to another. Not based on him doing anything well, but again based on the Cowboys not having any better alternatives.
Take a look at this video where Belicheck breaks down the Cowboys players, paying particular attention to #98:
When he's "noticed" it's for bad plays.
Like here, when he helps to give up a huge Barkely run: