I've wondered this myself and have spent a lot of time researching the decision process to understand what they look for. The truth is that not being accepted into a program does not necessarily mean you were not just as qualified as the admitted students (I hope everyone realizes this). After looking at GPA, Letters of Rec, GRE scores etc., the admissions committee is trying to create a diverse cohort. This is important to remember. They don't want every applicant to have the same background or same research/clinical interests. Your acceptance can really be influenced by the pool of applicants applying the same year as you. To be competitive, an applicant can attain strong grades, strong GRE scores, excellent statement, strong LORs, and clinical or research experience. What that same applicant cannot prepare for are the other factors an admissions committee at a particular programs needs that year. In sum, the committee is looking for fit. Not only fit with the program's need, but also how you fit with the other selected applicants.