What's the literary term for the fact that the audience and readers know of Mrs. Wright's motive in Trifles, but the sheriff and county attorney don't?

"Dramatic irony" is the term. Dramatic irony occurs in a work of literature (such as short story, a play, a novel, or a film) when the audience or reader is aware of a piece (or pieces) of information that one or more character in the story is not aware of.