Some of those countries are massively lower population density than urban United States (Mongolia), some are naturally isolated from other nations (New Zealand)

Some of those countries are massively lower population density than urban United States (Mongolia), some are naturally isolated from other nations (New Zealand), at least a few are probably lying (Cuba) and other situations. Of course there's also a level of irresponsibility in the United States and strong independent/anti-authoritarian views that make it hard in a pandemic.