Vaccine rollouts in most of the rest of the world--Europe included-- are far, far behind the USA. That might explain some of it.
Not sure about Michigan and some of the other states that are either increasing or not dropping, though. Warmer weather in the south maybe helping those states out, a bit?
I've heard folks speculate that the month of March might represent the largest temperature delta from northern states, to southern states, that we see through all of winter/spring/summer. So that might be contributing as well.