Huckleberry Finn is a boy, first and foremost, and he is also honest and innocent enough to be caught between his own sense of moral law and the civil laws of his society. This conflict is central to the nature of the text, making Huckleberry Finn's narrative perspective a fitting one for the story.
Huck is in a stage of his development where he understands the rules, laws, and mores of his society as far as what actions they prefer or mandate. However, he does not always find agreement with the notion that these are the best or "right" actions to take. He does not understand the reasons behind civil law.
We can see this conflict clearly in many sections of the novel, as we see it when Huck refrains from turning Jim over to two slave hunters.
On the one hand, Jim is fast becoming his friend. Jim has confided in Huck, relating to him his plans to buy his family eventually. On the other hand, Huck feels bound by the law, which states that it is a crime to aid an escaping slave.
In Huck's own words, appearing later in the text but relating to the same moral dilemma, Huck articulates the problems of his conscience this way:
"So I was full of trouble, full as I could be..."
As this trouble is at the heart of the novel's subject and themes, Huck's perspective on his dilemma is perfectly fitting.
Huck must decide which moral code he will adopt as an adult, his own or that of society. The novel goes a long way to suggest that society's moral code is far from flawless and is probably corrupted by the folly of adulthood, hubris, and accepted "wisdom".
I think Twain picked Huck as the narrator because he is still a child and was raised in the stereo-typical southern society. Also, the fact that he is young means he still has innocence. Therefore, his is able to look beyond Jim's color and become his friend. It reminds me of Scout from To Kill A Mockingbird. Hope that helped!
Huckelberry Finn was a great book. I think he chose to use Huck's POV (Point Of View) because he is the main character. Also he was having the problems with Jim the Slave, The Duke and King, and with Tom Sawyer.