The reason is power, and efficiency just happens to be a by product.
AC can transfer a heck of a lot more power than dc, that's why your 'friendly' power company transfers electricity to your local sub-station at +200,000 volts (AC).
If you ever look on the blades of a 120mm fan, the AC ones have a steeper pitch than the DC ones. The AC fan can turn at a lower speed, but pull more torque, and consequently more volume per rotation.