Why did Twain write in dialect

He used dialect for a number of reasons.

He could reflect the background of the character. "Ain't no way Jim's lyin'!" is a whole lot different than "Jim does not lie." A person who is educated would say the second. A person not so well educated would say the first.

He could come up with figures of speech that suited the character. "Solerman ain't no wise man at all, dag gum him. He didn't know no value for a child."

He could embed something funny in ignorance. He didn't often do that.