Muhammad Ali had an extraordinary effect on people. He showed immense personal courage during the Vietnam War and was willing to sacrifice everything to hold his principles. This inspired many and at the same time infuriated many.
When he returned to boxing, he may have lost a bit of quickness but none of his courage nor his ability to infuriate people.
Take George Foreman and Joe Frazier. He made his legendary matches with them highly personal, insulting their looks and their intelligence. Frazier in particular was enraged and after one of their fights, Ali said it was the closest he had ever come to dying.