Most songs are heard multiple times, by various singers. A poetry reading 'usually' presents newish material each time. It may be difficult to absorb the sense of either a song or a poem in one hearing..... thus poetry is more often relegated to printed medium and read privately by someone who already appreciates the poetic form.

That may not explain everything, but I'd bet that decent poetry, read to ones self, is then easier to comprehend when then hearing it read aloud. (I heard Alan Ginsberg read aloud some of his stuff, and he really did make it more meaningful than just in printed word.)