DC is also used because you avoid grid synchronization issues, and this is generally the trend for long distance connects now in general. It means the grids can have frequency divergence without affecting the transmission line itself.
If I remember correctly that actually makes things much worse if you've operated at one potential for a considerable amount of time.
You could operate the cable at AC ~ 0.1Hz but your ability to actually transmit MWH would suck.
Whatever, the ground currents penetrate very deep into the earths crust thus equally effecting buried cables.
Its the difference between just shifting the frequency or phase on your local control clock at the substation, and trying to coordinate with another state/territory/nation's entire AC generation infrastructure to keep everything in phase.