The New York Times has a good animated infographic the "traced the hidden spread of the epidemic to explain why the United States failed to stop it. At every crucial moment, American officials were weeks or months behind the reality of the outbreak. Those delays likely cost tens of thousands of lives."
Restricting travel from China helped curb the spread at first, but infected travelers came in from all other parts of the world in February, leading to additional outbreaks. Tests developed by the CDC didn't work, which meant crucial tracking and tracing didn't happen. In late February and health experts recommended social distancing measures, but Trump ignored his health advisors and told Americans they should continue to fly, shop, work in offices and factories, and go about their normal lives. On March 2, New York mayor Bill de Blasio said, “I’m encouraging New Yorkers to go on with your lives and get out on the town.” At that point, a pandemic was inevitable.
