Why do people think Mark Twain's Huckleberry Finn is racist?

It blows my mind that people accuse Mark Twain of racism, when he has written such a great book that exposes the moral injustices and cruelty involved in slavery and its society. I think he had to portray all the racist ideologies to send the message and make an impact on the reader. So why do people think Mark Twain is racist because of Huckleberry Finn?