Latency combined with high contagion rate during that period.
A WSJ article opined that the US hit the worst possible approach to this, a shut down that wasn't long enough, and just bulling through it with current precautions would have been a better compromise (a la Sweden). France shut down and reopened and is shutting down again partially because the rates are going back up. And they start back to K-12 school today.
NY state, hit hard early, now is way down. States not hit hard early went up, and now are coming down.
Area under the curve.
When did we last have a flu season with 170,000 deaths?