It isn’t; neither, in and of themselves, is more or less efficient than the other. It really depends on the application.
Traditionally, in grid-scale power distribution, AC is traditionally considered more efficient because it can _easily/cheaply_ be stepped-up to a higher voltage (with a simple transformer), which decreases the current in direct proportion, and therefore dramatically lowers the power loss in the copper conductors separating a power station from homes (however this is changing).
I most other applications, one is not considered more efficient than the other, they’re simply used where it’s appropriate.
AC could be considered more efficient for distribution because it is easy to step up to high voltage levels (=> lower current for the same power transmission => lower resistive losses) using a transformer. However, for very long power transmission lines, at very high voltages, capacitive coupling of AC voltage to the earth, and skin effects can cause some additional losses. High-voltage DC links are used in a number of places for point-to-point transmission.