Pickett plays the run stuffer role in the 4-3 defense. The Bills have Marcus Stroud in that role already.
The Packers don't need him in their new 3-4 defense, Raji will play nose tackle and Pickett doesn't have the speed or athleticism to play DE in a 3-4 scheme.
The Bills are in need of the penetrator type of DT (think Tommie Harris of the Bears) Triplett was that type of undersized speed DT, but he was already washed up when the Bills signed him. They hoped McCargo would be that guy as well, but it's not panning out.