Vaccination rates would be my first guess as well. Individually vaccines reduce the severity of symptoms, which makes it less likely for people to notice and report that they have COVID. The vaccine also reduces the viral load spread by individuals, therefore making it less likely for them to spread the virus, and it also makes it less likely for you to become sick if you come into contact with the virus.
Considering the difference in population density between the USA and The Netherlands (NL is way denser) I'd say this is a good demonstration of how vaccines are miracles of modern science.