Why does including race or gender as a factor in the evaluation of a candidate exclude ability? All hiring is based on a variety of factors, not just 'ability'.

Availability for the position, quantity of experience, leadership skill, willingness to learn, familiarity with the problem space, quality of personal references, interpersonal skills... all that along with 'how good are you at writing code'. What is the huge negative impact of adding 'diversity' to that set of evaluation factors? Are you arguing that adding diversity would actually push the most qualified candidates to the bottom of the pile somehow? If all things are truly equal, adding diversity to the sort has no impact on the ability of qualified candidates to get hired.