To put it most simply, the oral traditional idiom persisted because—even in a written incarnation—it offered the only avenue to the immanent poetic tradition, the invisible but ever-present aesthetic context for all of the poems.
"Texts That Speak to Readers Who Hear: Old English Poetry and the Languages of Oral Tradition", in Speaking Two Languages: Traditional Disciplines and Contemporary Theory in Medieval Studies, ed. Allen J. Frantzen (1991), p. 155
We know now that cultures are not oral or literate; rather they employ a menu or spectrum of communicative strategies, some of them associated with texts, some with voices, and some with both.
"What's in a Sign?", in Signs of Orality. The Oral Tradition and its Influence in the Greek and Roman World, ed. E. Anne MacKay (1999), p. 3
The default designation of poetry has become written poetry. That's why we have to prefix the adjective "oral," because the unmodified noun no longer covers anything but written poetry. That's also why we resort to other unwieldy phrases to pigeonhole events and phenomena that our cultural proclivities have silently eliminated from consideration. Thus a "poetry reading" describes a performance (from a published text, of course) before a well-behaved, often academic audience. Thus "spoken-word poetry"—so redundant from a historical perspective—identifies voiced verbal art, verse that is lifted off the page and into the world of presence and experience.