The Medicare program offers an option called Medicare Advantage in which private insurance plans compete with traditional Medicare. The Affordable Care Act set up marketplaces in which insurers compete for enrollees. Both programs provide government-subsidized health insurance coverage in a regulated market that encourages private plans to compete for market share based on premiums, quality, provider networks, cost-sharing, and benefits. Yet the Medicare Advantage markets are substantially more robust, with higher private insurer participation and lower average premium growth. We assess how structural differences between Medicare Advantage and marketplaces led to these outcomes, and provide recommendations for strengthening the marketplaces.