Why is the conclusion of The Lottery, a surprise?

Concerning the surprise ending of Jackson's "The Lottery," the ending is accomplished primarily by the use of an objective, detached point of view.  The people are acting normally in a normal-looking town, and that's all the information the narrator reveals to the reader.  Any other point of view would reveal too much information to maintain the surprise ending.  The lack of any thoughts being revealed, as well as the lack of any authorial intervention or explanation, enables the surprise ending to be effective.

Also, a small point, but I would refer to the stones being piled up, etc., as foreshadowing, rather than hinting.  Hinting implies that the writer is trying to get the reader to guess--in this case, to guess what's really going on.  And that's the last thing the story could afford.  Foreshadowing, in contrast, is not hint giving.  Foreshadowing gives the ending legitimacy once it occurs:  once the true nature of the lottery is revealed, it makes sense because the pile of stones and other details have previously been revealed.