Another aspect is that not every Ph.D. graduate wants an academic position at a major research university. Some are just not willing to take on the pressure of that kind of job in the current funding climate. I have had both former students and family members make that choice.
@juillet is absolutely right that it is important to make an offer to the candidate whom you think has the best chance of success at your university. Not many universities (with the possible exception of the most selective) are willing to invest a lot of money (particularly with the high cost start up packages in science and engineering) in a faculty member with the expectation that the success rate is below 50% in being tenured eventually. This means that you only have your applicant pool to deal with and if you don’t get underrepresented candidates who are competitive, then there is pretty much nothing to be done despite the desire to broaden the representation in the department.