Defensive metrics measure accomplishments, not ability, despite what many fans think.
Bradley was among the top in innings for CF, but his chances, putouts, etc. were no commensurate with his innings played. Basically, he saw fewer opportunities per inning than a lot of other CF. So the rest got credit for making plays on opportunities while Bradley did not, and therefore his overall metrics suffered.
His metrics were not a knock on his ability to play defense...