What we came up with regarding why they were successful was this:
1) They're short
2) They do create conflict--in the *reader.* Not just an "Aha!" moment, but an eagerness to understand, a frustration (My own idea-story, "Schrodinger's Cat Chases the Super String," is about the unknowability of the universe in opposition to humanity's desire to keep kicking this thing until we figure out how it works), a sorrow, a denial, a realization.... could I walk away from Omelas? do I really want to know God that much? Whatever.
3) They get out immediately after the punchline. Smack you in the nose once ,and leave you blinking before you come back.
There's some of these in literature, too. "The Lottery," "The Lady and the Tiger," your average O. Henry story.
It's kind of a cool thing to figure out.
Whereas widget stories are essentially, "Here is a cool widget. It goes bleep."