My hypothesis is that he did it to minimize the division in our society between the maskers and anti-maskers.
President Trump was in a hospital when he wore the mask in that picture just reposted here, wasn't he? That at least is a somewhat more typical/normal situation in which to wear one.
When I am with certain family members I will briefly wear a mask such as to go into an eating establishment rather than stir up conflict with people who possibly need truth on other subjects first.
When in other situations I am more adamant about not wearing one. I have denied my "mandated mask" city and county a lot of sales tax revenue by going to businesses in nearby counties that do not require masks.