Because he's not guilty. He is presumed innocent until a judge, or twelve good folk and true, find him guilty beyond a reasonable doubt. The defense attorney's duty is to ensure the state proves the crime. The defense attorney is prohibited, however, from promoting fraud upon the court. He cannot introduce testimony or witnesses that he believes to be false. He can't argue that his client is innocent. He can negotiate with the prosecution for a better deal. He can fight to exclude evidence. He can challenge evidence in court. He can erect flaming hoops for the prosecution to jump through.
That's not to say that all defense attorneys live up to that standard.