Simple, because it's so prone to self-transformation. It changes all the time. It responds to the atmosphere. The space, the mutual space of interaction between audience and stage. And no two performances are ever the same.
Theater can respond immediately.
Some people call it living theater, some people call it newspaper theater. Whatever it is, street theater, it can respond immediately to both events and the changing pattern of events. It responds to the dynamics of any situation. It's the tension between the performers on stage and the audience watching, where the hope for some kind of a transformation comes about. That nebulous territory which is constantly being traversed by lines of force from both sides.