The “textual history” (the physical text) answer is that Aristotle’s piece on comedy did not survive into the Renaissance (perhaps the library fire at Alexandria was responsible), but was lost in the intervening centuries.  But a more penetrating answer is that Aristotle saw Poetics as a form of Rhetoric, as owing its existence to the argumentative social power of words.  His analysis, for example, of catharsis dealt with the effect on the audience of tragedy’s unfolding—the “lesson” (sociologically and religiously) taught by tragedy, which helped solidify and unify the social morality.  To say that he gave tragedy more importance than comedy may be arguable, but is by no means a given.  Later critics have examined comedy for its rhetorical value as well (Elias Canetti, in Crowds and Power, for example, says that Laughter is baring one’s teeth at the enemy).