Related: I wasn't in the US at the time, but I think a close 6) would have to be the intense polarization and ongoing "culture war," wherein one side was already being told that something else coming out of their mouth was literally killing people, and doubting the veracity of the statement. Wearing a mask is slightly panic-inducing anyway (ever try to muzzle a dog?), and coupled with the fact that one side of the political isle took up its championing, this allowed some members of one side to wear the mask sanctimoniously and some members of the other to avoid or disbelieve any rhetoric (scientific or otherwise) promoting their use. As long as the more general lack of cooperation carries on, the US is at risk of not being able to unify over important things like public health.