1) He clearly considers himself a pretty boy judging by the amount of make up he douses himself daily with so …. simple vanity could be one reason.
2) He has been activating his de facto storm troopers lately with his inflammatory macho dumb shit which they love and respond to. He's had them screaming and waving their Ar-15's at various statehouses where governors have the nerve to disagree with him. A mask would send the "wrong signal" to his storm troopers. A sign of "weakness" that may erode his control over their feeble minds.
What do you think …… WHY DOESN'T DONALD WEAR A MASK?
One reason I threw this up here was to balance trump's latest diversion …. Obamagate. BTW did you hear that Pelosi had intercourse with Godzilla? It's true ….. I read it on the internet.