Why is Huckleberry Finn considered the realist of the story?

In Mark Twain's time romanticism (books about adventure, exotic places, extreme emotions, etc...) was popular. Twain was interesting in that although he was critical of romanticism, he also wrote it in the form of his stories about Tom Sawyer. This novel contrasts the different values of realism and romanticism through the characters of Tom and Huck. Tom is a romantic. He wants to create a gang of bandits to go on adventures like he has read about in his romantic novels. At the end of the story he even creates an elaborate plot to rescue Jim, even though he knows that such a plot is entirely pointless. This ends up getting him shot. 

Huck, on the other hand, is the realist. Huck is less interested in glory or adventure. He leaves home not for adventure but to escape his father. While he naively believes some of Tom's stories, as the story moves on he starts to see things from a more realistic perspective. For example, he aims to stop the Duke and King from pulling their scam. This is because he sees them for the criminals they are. Tom might have gone along with the performance, simply because it was exciting. While Huck is superstitious and naive at times, he does almost everything he does for practical purposes, not because he is recreating something he has read in an adventure story.