How and why are Mark Twain's forms of writing and content considered to be "American"?

Mark Twain’s writing might be considered
characteristically “American” in a number of ways, including the
following:

• his focus on American
  people, places, and other American subject-matters, as in
  Adventures of Huckleberry Finn, The Adventures of Tom
  Sawyer, The Gilded Age, etc.,
  etc.


• his tendency to look at Europe and the
  rest of the “old” world from a skeptically American perspective, as in
  such travel books as An Innocent Abroad and in such works of
  fiction as A Connecticut Yankee in King Arthur’s
  Court


• his emphasis on “frontier
  humor,” as in his famous story “The
  Celebrated Jumping Frog of Calaveras County,” among many other
  works


• his frequent tendency to use
  varieties of American dialect, most memorably in Adventures of
  Huckleberry Finn


• 
  


• his skill in capturing the sounds of the
  language spoken by many African Americans of his day, such as in the
  following passage when Jim, in Huckleberry Finn, tells Huck how he
  once slapped his very young daughter because he failed to realize that she had become
  deaf and thus couldn’t hear his commands:

readability="16">

“Oh, Huck, I bust out a-cryin en grab her up in
my arms, en say, ‘Oh, de po’ little thing! De Lord God Almighty fogive po’ ole Jim, kaze
he never gwyne to fogive hisself as long’s he live!’ Oh, she was plumb deef en dumb,
Huck, plumb deef en dumb – en I’d been a-treat’n her
so!”

This is one of the most
moving passages in the entire book, and the fact that Twain can create such moving
speech by using such allegedly “unsophisticated” language is one of the great aspects of
his genius.

• his habit of
  celebrating American democratic values, American habits, and the American
  people, especially in contrast with the values, habits, and people of
  Europe (particularly the French).


• his
  willingness to criticize America when he felt that it had fallen short of its own ideals
  or of ideal behavior in
  general