Add the race factor into it, people who hate to see young rich flashy Black men view him as a heel and watch his matches to see him get beaten up. They will pay in hopes of seeing him lose. Even people who aren't boxing fans.
Mayweather exploits this anger/resentment of rich and "arrogant" Black men to drive PPV sales to his fights. Laughed all the way to the bank and got people to pay for a fight of a mixed martial artist boxing for the FIRST time against an undefeated all time great boxer. hahahahahahahahaahahahahah