Musing about this, I have the impression that the rise of plays is simultaneous with the dying out of epic poetry. Shakespeare did both, and playwrights were respected less than poets at the time (and plays heavily censored and restricted by law), but as time passes, more playwrights and fewer epic poems, until by the 1900s, I'm hard-pressed to think of any long-form poets besides Eliot and Pound - though playwrights are still going like gangbusters with the likes of Tennessee Williams both commercially & critically popular.