It's more efficient for very long distance transmission.
With AC transmission at 60 Hz, the wavelength is 5000 km. Things get complicated when you approach that distance.
Also, with maximum peak voltage limited by arcing, DC uses the wires more efficiently because it's constantly at peak voltage, rather than oscillating up to it.
Also, AC current creates a varying magnetic field that pushes the current flow away from the center of the wire, so only the outer region gets used, increasing resistive losses. It's not a huge effect at 60 Hz, but it makes some difference.