Columbus for what? What did he do to deserve glory? He found some land, tortured and killed people to rule it. His rule led literally to millions of people dead.
MLK was a leader of the Civil Rights Movement that lead directly to ending Segragation and the Civil Rights Act of 1964. He helped literally millions of people.
Sure sure sure... standards of the time. But what is better about finding some land than helping millions of people gain equal rights?