The article does state this reason. > A second global flu epidemic in 1934 cemented Japan’s love affair with the facemask, which began to be worn with regulari

The article does state this reason. > A second global flu epidemic in 1934 cemented Japan’s love affair with the facemask, which began to be worn with regularity during the winter months—primarily, given Japan’s obsession with social courtesy, by cough-and-cold victims seeking to avoid transmitting their germs to others, rather than healthy people looking to prevent the onset of illness.