I don't know if you can ever say why something didn't win an award, or a prize. To some, the answer would be that the one that lost wasn't as good as the one that won. You also have to take into consideration that judges on the panel and their own tastes and opinions.
Here are the judges in the Drama section:
Ben Brantley, chief drama critic, The New York Times (Chair)
Kimberly W. Benston, Francis B. Gummere Professor of English, Haverford College, Haverford, PA
Karen D’Souza, drama critic, San Jose Mercury News
Rohan Preston, theater critic, Star Tribune, Minneapolis-St. Paul
- Paula Vogel, playwright and professor of English, Brown University