Many schools make their admissions decisions by using a point system. This means that after you are interviewed, your interview performance, your essays, your GPA, your MCAT score, etc will each count for a certain number of points depending on how good they are. Once all of the points are assigned, the school will make a decision on you.
In general, a very high overall score will warrant an acceptance, a very low score will warrant a rejection, and those in the middle are placed on hold or waitlisted. The cutoffs usually depend on the scores of the students who were accepted during the previous cycle.
You need to remember that schools interview many more students than they can possibly accept, and a waitlist of several hundred people just isn't good for anyone. Therefore, schools cut the applicants at the bottom loose after an interview.
I'm not saying that this is true of all schools, but at many, if your interview, your stats, etc. don't add up to enough points...you get the axe.