First, the real question is why much (if not most) of the frum community tends to be skeptical of masks. That warrants a serious answer.
1. For the same reason many goyim are skeptical. The announced that Covid19 would be something similar to Ebola, Smallpox, Bubonic Plague, or at least the (misnamed) “Spanish Flu” of 1918. In terms of key factors the distinguish a serious public health threat, from what is a normal fact of life, Covid19 was a flop. It has almost no impact on children (i.e. less threatening than the annual flu). Antibody tests proved that over 90% of the cases don’t result in serious illness. The death rate increased (in the USA) from 0.8% to 0.9% (well within a “normal” range, note that Israel is about 0.5%, and many European countries are around 1%). And by counting people who test positive but don’t get sick as being “Covid19 cases” it looks like they are fudging the statistics. But this applies to goyim as well as Yidden.
2. Frum Jews are quite young since we have large families, and unlike real epidemics (and similar to normal diseases), Covid19 tends to kill older people. Look at the typical goy as someone almost ready for “assisted living”, and the typical frum yid as being someone in heder – they are dropping dead, and we aren’t.
3. Most Covid19 mortality is of the generation that is older than boomers, but too young to be the “qreatest generation” (i.e. who were children during World War II). These would be roughly those born between 1930 and 1946. Among us, these were the children of the holocaust, too young to work or hide or fight, and consequently unlikely to have survived the war. Most frum Jews who “should” be dying of Covid19 based on their age, never survived childhood. Among Americans, no children died due to hostile actions during World War II (e.g. so people like Joe Biden lived to be old and needing to worry about Covid19, a Jew born in Europe during the war, probably died in childhood).
4. Jews, like Blacks, have has long, very negative history, with public health figures. In the 20th century, in both Europe and America, public health “experts” believed in the then respectable and accepted science of Eugenics, which was the intellectual basis for such things as the holocaust, “Jim Crow” (and earlier, for slavery), immigration restrictions, and coercive family planning policies. Now the same public health community that thought that restricting the number of Jews was a necessary public health measure, are claiming that Jews davening, learning and doing mitsvos is a public health menace. We’ve heard that before.