https://en.wikipedia.org/wiki/List_of_presidents_of_the_United_States_with_facial_hair#Overview
Facial hair fell out of favor for health reasons, as described on the PBS website pertaining to a documentary on tuberculosis: "Most men at the turn of the [twentieth] century featured stylish beards or mustaches, but showing off a smooth face became a new trend once public health officials maintained that men could transmit dangerous infectious particles through the scruff of their facial hair..." [...]
The last major party candidate for the presidency to have any facial hair was Thomas E. Dewey, who had a mustache, and was defeated at both the 1944 and 1948 presidential elections. Some pundits even claimed that public disapproval of Dewey's mustache may have contributed to his two electoral defeats.
Social scientists have researched the effect of facial hair on the electability of presidential candidates, and as of 2010 currently consider facial hair to have a negative effect on candidates. The existence of facial hair on potential presidential candidates is regularly noted (albeit somewhat jokingly) as a harmful factor.