Twain was a contemporary of the times of Smith and he couldn't understand why it had the King James prose.
To this day Mormons pray in King James prose. Because it's respectful? Which leads to the question. What do you pray in if you are Japanese, German, Spanish etc?
Twain in the book Roughing It points out that the BOM condemns polygamy. Also he writes that Joe Smith didn't practice polygamy. And that Brigham did. Twain also dedicates many pages to Brigham Young and The Mountain Meadow Massacre.