Regardless of what life dealt Willie or Hank, the test of a man is what he does with adversity. Willie let it turn him into a dick.
I was a Mays fan as a kid when he returned to NYC with the Mets, and I was so excited when I got to finally meet and greet one of my boyhood idols at a card show many years later. Just like so many other boys in mens' bodies, I worked up my courage and tried to express how much Mays meant to me and how honored I was to meet him. He said...nothing. He didn't even acknowledge my existence. Just rolled the signed baseball down the table to the handler. Felt just terrible; I paid someone to insult me. Well, I sold off my entire Mays collection including that Willie-Mickey-Duke signed baseball.
I met Hank Aaron around the same time at another show. His flight was late from Atlanta and he arrived at the show three hours late. He thanked the group for staying around to get his autograph, and when I got up to the front of the line and fan-boyed about how I idolized him as a kid he looked at me and actually spoke to me graciously. I still have the card and ticket, and will until I die:
Anyone who thinks Hank Aaron had it easy in the South Atlantic League and in the Jim Crow years is nuts, not to mention the abuse he endured during the home run chase. What he did with his experiences is the difference between him and Mays. Willie was probably a better ballplayer at his peak but he was never as good a person.
In the end, it does all come down to basic personality traits. In terms of fame, Aaron and Mays do not compare to Muhammad Ali, yet Ali was absolutely the best to his fans because he was a gregarious, outgoing man. You never hear stories of Ali being rude, ignoring kids, etc. Quite the opposite. Same with Babe Ruth.
I realize this has gone far afield from cards, but it does explain some of Mays' lack of popularity for collectors. You have to acknowledge his greatness but the rest of the package doesn't leave you feeling good. Take Roberto Clemente as a contrast. Clemente died heroically, which cemented his legacy among collectors and makes his stuff more desirable than it should be given his relative place in the rankings of great players (39th in career WAR; Mays is 5th, Aaron 7th). Yet as a man, Clemente was someone you'd want your son to emulate.