Yet I haven't seen anything that tells me why.
It seems to me that the largest publishers would have paid him a personal visit to convince him to publish his books with them, but they remained with his own publishing company until the company went out of business in 1894. It experienced financial failure. Turns out, he published his own books, which did quite well, and his friend's Ulysses S. Grant's autobiography, Personal Memoirs, which also did well. But then he published a number of other autobiographies, including Tecumseh Sherman's and the Pope's, which flopped.
In Mark Twain's autobiography, he said, "All publishers are Columbuses. The successful author is their America. The reflection that they--like Columbus--didn't discover what they expected to discover, and didn't discover what they started out to discover, doesn't trouble them. All they remember is that they discovered America; they forget that they started out to discover some patch or corner of India."
His life was fascinating, especially considering the personal tragedy he faced while still churning out humorous vocabulary.
What also struck me during the biography was Arthur Miller's comments about why Mark Twain was never a good businessman. He said (and I am paraphrasing) that creative people such as writers can see the possibilities in an invention or a course of action; they have a vision that others might not share. But perhaps because they see the possibilities, because they have that vision, they often don't see the side that businessmen look for: the fatal flaw in the plan, the financial aspect, the potential for failure.
Do you like Mark Twain's books? Which is your favorite?