Diversity of interests wasn't always a priority in college admissions though. Malcolm Gladwell has a good piece on how the current Ivy League admission focus on non-academic indicators was originally a racially driven policy. One could make a good case that "diversity of interests" is a policy put in place to ensure that highly performing performing demographic can dominate admissions. I imagine colleges con't want that to happen because it would jeopardize the long-term health of their institution.

Malcolm Gladwell's article is here: http://www.gladwell.com/2005/2005_10_10_a_admissions.html