"Huck Finn" Movie Preserved Forever

The National Film Preservation Board announced that a 1920 version of "Huckleberry Finn" will be the 1000th movie they preserve. That's great, though of course there are easily 100,000 movies to go that need preserving and so many have already been lost to time and neglect. But it raises a question: why are there so few decent films based on books by Mark Twain? He's certainly one of the greatest -- if not the greatest -- American writer and his body of work is vast and varied. I've certainly never seen any adaptation of his scathing, brilliant, dark masterpiece "The Adventures of Huckleberry Finn" that came within a country mile of its complexity. "Finn," "Tom Sawyer," "The Prince and the Pauper" and "A Connecticut Yankee In King Arthur's Court" seem to be the ones that get remade again and again and again. Some are decent, even good, but I don't think any of them are great. Is Twain not suited to film? Is his wit too pointed? What gives?