I hadn't heard that before, and it explains a lot. Apparently Twain planned for the story to be about Huck Finn's development into an adult.

He worked on it for years, then gave up on it for several years, and only after making a trip on the Hudson River did he decide to make this book about an adventure travelling down a river instead. Still doesn't explain the contrived ending though.

I never did think this book was anything special. I guess it was creative and influential for the time, but it doesn't hold up well compared to other books from the time, including Twain's other work.