Magnetic monopoles

Why is it that electricity can exist as a negative or positive charge, but magnetism is always paired with north and south poles? A setup was created to detect magnetic monopoles by producing an electric current as they passed through. This single detection has never been reproduced, so the existence of monopoles remains in question.
 Blas Cabrera United States 1982 Magnetic monopoles

Why is it that electricity can exist as a negative or positive charge, but magnetism is always paired with north and south poles? A setup was created to detect magnetic monopoles by producing an electric current as they passed through. This single detection has never been reproduced, so the existence of monopoles remains in question.