Because he knew he had to sell himself. And you can't do that with limited vocabulary if you're going to do it verbally.
Still a damn stretch from speaking well but as pointed out he certainly spoke better than his modern day contemporaries. There is a reason for this but most of you will not like or understand the reasons.
Ali was way ahead of his time in an era of dumb ignorant people who judged people by the color of their skin
Ali was obsessed with professional wrestling and understood how to manipulate crowds into liking or hating him. Hell, sometimes you don't even need to be smart to flaunt the charisma, look at Floyd playing the roll of mega villain, or Conor McGregor...