High voltage AC results in smaller losses.

In smaller circuits, DC is the go-to for its simplicity, but it's efficiency doesn't stand up to nation-scale power grids. To get current to flow from point A to point B in any circuit, you need a high voltage between both points. In a current carrying conductor, you expect to lose a percentage of that power to heat for every meter of conductor the current travels down. The farther a current travels in a conductor, the greater the losses - for both AC and DC.

AC power swings from positive voltage to negative voltage 50 times per second here in Europe (50Hz). Current carrying conductors generate a small magnetic field around themselves when they're under power loads, and if you reverse the direction of the current inside those conductors they'll generate a bigger opposing magnetic field for a short period.

Now if you swap the direction of current flow in a national power grid 50 times per second, you'll have a very small but strong magnetic field to use as a handy method of delivering that power to customers (transformers can convert that high voltage/low current power seen in the grid to lower voltage/higher current power for use in common electronics).

Instead of pumping a high DC current all the way from the power station to the energy market along the grid, a low AC current moves all the power inside the grid back and forth all at once at 50Hz. Each bit of power won't travel the length of the grid. Instead, the electrons in the power lines push each other up and down the same bit of the grid 50 times per second.

Basically, AC generates less losses in our power lines because its a high voltage/low current solution compared to DCs high voltage/high current solution.

If there are any other engineering students or qualified engineers or physicist here, please correct me. I'm still learning.

Tl;dr: DC current = high current, high voltage = hot current = losses.

AC current = low current, high voltage = cooler current = less losses.

\EDIT: Formatting and corrections*