Call me crazy but I think the United States' particular history stunts it in poetry. It has no tradition of monarchy and aristocracy and it's a thoroughly low-church Protestant nation; it hates Catholics, the Orthodox are nowhere in sight, and even the Anglicans never truly fit in here. Poetry is a high art, especially the great epic modes. America has never been a "high" nation, not in its soul.
Even guys like Milton, over in Europe, never truly escape the long shadow of Europe's history of the nobility and the Church.