Because not all people gravitate towards the same kind of work. People will react to someone's writing in different ways. They have experiences and preferences and preconceived notions that shape the way they react. And people's moods change the way they read something. They're reading hundreds and hundreds of pages of writing. It's reasonable that they would tire of a certain subject / style if the past number of samples all just so happened to follow the same style / content. And then there is the space issue. If they accept 5, and the waitlist is 5, still, that is just 10. The top 10 of 500 (or more) applicants. It's also reasonable to believe that committees make their offers based on the likelihood of an applicant's chance of going to their school. It's reasonable, I think, for a school to pass over an excellent applicant for a lesser so applicant that is more likely to accept. So they are any number of reasons (didn't even touch on the SOP and LOR factors), one's we can probably never know, unless we are on one of these committees ourselves.
Lesson I learned during this application season: Rejections don't invalidate my work. Also, acceptances don't necessarily validate my work either. I still have much to improve.
Edit: Forgot to mention -- if anyone is waiting to hear from Vanderbilt, they are notifying applicants tomorrow!