Plot is the soul of tragedy because it is where the emotion, and heart, come from. We feel tragedy because of the exciting things that happen. They are often sad and exciting, that's what makes it tragedy. Without those events, it's just another story.
Poetics, Section 1, Part VI
most important ... 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, ... if you string together... character, ... diction and thought, you will not produce the essential tragic effect nearly so well as [a play that] has a plot and artistically constructed incidents. The plot, then, is the first principle, and, as it were, the soul of a tragedy; ....
Aristotle is enumerating the five parts of drama and comparing their effects upon a successful tragedy. For example, he says that even with finely wrought characters and finely tuned diction and thought, there will be weak tragedy if the actions of the plot are weak or inartistically constructed. He emphasizes that action is the important element that brings about tragedy. Therefore, since action--and action is plot--creates the tragic events and the tragic effect, it is plot that is the central feature, the driving force, the "first principle," "the soul of a tragedy." [In the quote above, Aristotle clearly equates action with plot.]