“Why the difference?”
Our outbreaks are in new areas of the country. We’re a big country.
An outbreak seems to run it’s course.
Ny, etc, are seeing about the same as Europe.
Or, it could be that Covid is being subject to natural selection/mutating. Threatening to kill its host, so the host does everything it can to defeat it, is not an optimal strategy. Far better to not kill the host/cause no symptoms, such that the host takes no action against it. Then it can propogate widely.