The short answer is that a jury acquitted her after an hour to an hour-and-a-half of deliberations, so we don't know for sure, but presumably the jury found there was reasonable doubt as to whether she had done it. [Mary Cantwell, "Lizzie Borden Took an Ax", The New York Times Magazine, July 26, 1992.] (Cantwell's article also suggests that maybe they just didn't want to hang a woman, but that would be speculation, the same as wondering which particular bit or bits of evidence may have caused the jury's reasonable doubt.)