It really just comes down to the difficulty in assessing candidate capability and predict future performance. Couple that with pressure from executive to improve diversity numbers, reinforce it with bonus tied to hitting diversity numbers, now there's a lot of incentives to hire minority candidate that's good enough and pass on a stronger, non-minority candidate (better by how much, who knows? Interviews are notoriously unreliable indicator of future job performance anyway). :P
Edit: note that if the minority candidate is as strong or stronger than non-minority candidate, it is a no-brainer decision and win-win for all involved. :)
Edit: note that if the minority candidate is as strong or stronger than non-minority candidate, it is a no-brainer decision and win-win for all involved. :)