In The Adventures of Huckleberry Finn, what are two reasons Huck does not immediately reveal the deception to Mary Jane or the Doctor?
I think we need to remember the position of power that the Duke and the King had on the raft, and how they were able to dominate both Jim and Huck. Huck was only a boy, of course, and he also wanted to ensure that the Duke and the King did not turn in Jim as a runaway slave.
However, more profoundly perhaps, what is interesting about his time with the Duke and the King is that Huck seems to take the role of observer of human folly as he watches both the Duke and the King fleece a series of people, but also the tendency of people to ignore the rational element of human nature and to be tricked. Consider how in Chapter 25 the Doctor adds the one note of sanity to the proceedings, but is promptly ignored by everyone, because they want to believe that the Wilks have returned.
Another aspect you might want to think about is how Twain uses these episodes to show Huck's moral maturity. As he is enabled to see the antics of the King and the Duke up close at first hand, he shows a strengthening sense of what is right and wrong, and although Huck doesn't denounce the King and the Duke at first, he goes on to engineer the downfall of this particular scheme. Consider this reflection of Huck's on the proceedings that he is observing:
Well, the men gathered around and sympathised with them, and said all sorts of kind things to them, and carried their carpet-bags up the hill for them, and let them lean on them and cry, and told the king all about his brother's last moments, and the king he told it all over again on his hands to the duke, and both of them took on about that dead tanner like they'd lost the twelve disciples. Well, if ever I struck anything like it, I'm a nigger. It was enough to make a body ashamed of the human race.
This, then, represents Huck's continuing moral growth as he clearly feels deep disgust at the Duke and the King and how they act.