There was a lot of hype when the Patriots made the move for Bennett, but the signing of Jamie Collins really changed up the whole scheme. There is no doubting the skill of Bennett, but the switch to a 3-4 scheme really diminished his role.
Bennett just came off a week suspension given by the team for "philosophical differences" with his position coach, which likely started the snowball move to Dallas.
The Patriots gave up a 5th and 7th rounder to Philly for the defensive end, so the drop off isn't too severe, just a player that didn't pan out. Bennett will have his revenge game against the Patriots on November 24th in an already hyped-up game with the Cowboys.