Because Aristotle defined tragedy as mimesis, loosely translated as the "imitation of action," and this action must be either probable or necessary, and it must have serious implications and be complete.
The primary principle of tragedy, given Aristotle's argument that tragedy imitates action, is the plot, which he defines simply as how the author arranges incidents--not the story itself but how the author presents plot elements to the audience:
But most important of all is the structure of the incidents. For Tragedy is an imitation, not of men, but of an action and of life, and life consists in action, and its end is a mode of action, not a quality.
Aristotle goes on the elaborate on the nature of a successful plot: it must be complete, that is, have a beginning, which begins a cause-and-effect chain of events; a middle, essentially the high point in the action that precipitates the following action; and the end, which finally provides a resolution to the problem created at the beginning of the plot.
Perhaps the most important element of a good plot is that one action inevitably and logically leads to another action in a linear fashion. The audience should be able to follow the plot from the beginning to the end, with no episodic elements thrown in to disturb the linear development of the plot.
According to Aristotle, characters in tragedy were of secondary nature to the plot--they support the plot but they do not, in and of themselves, determine how the plot progresses.