Because she's a girl.
http://www.npr.org/sections/thetwo-w...se-shes-a-girl
Emily Nash was allowed to play. She just wasn't allowed to win.Nash, a junior at Lunenburg High School in Lunenburg, Mass., had the lowest score in the Central Mass Division 3 Boys' Golf Tournament. But the first-place trophy was awarded to a boy who was four strokes behind her, because of the rules of the tournament.
According to the Massachusetts Interscholastic Athletic Association, girls can play in the boys' golf tournament as part of a team, but they aren't allowed to be entered as individuals.