What makes you think that the Jazz WANT to be average or better defensively? Coach Hardy does not want to be good defensively: he shoved two best defenders, Kessler and Dunn, into the limited-minutes backups and never disciplined rookies (Keyonte and Brice) for not even trying to give an effort on defense. Ainge also has no interest in defense: he traded away Fontecchio, let Dunn walk away and signed a defensive sieve Mills. He also consistently drafts players who are bad defenders like Keyonte, Brice or Filipowski.
The Jazz have not become a horrible defensive team by accident: it was the result of concerted efforts by the front office and the coach. Young Jazz players are brought up in the environment where the defense clearly does not matter and they are taking it to heart. The Jazz will not become an average defensive team in the foreseeable future.