Question

why is direct current used in electroplating

Answer 1

Plating requires ions to flow through an electrolyte in an electric field.
Positive ions flow with the electric field, negative ions against the electric field. This electric field comes from a DC supply connected to the electrodes.

With AC, there will be no net ion flow and no plating will happen because the electric field direction will keep alternating and ions will oscillate back and forth within the electrolyte.

Some people incorrectly refer pulsed DC, where direction of current doesn't change but amplitude does, as AC. If you mean that, there no harm in using pulsed DC as long as the frequency is much higher than 1/time-constant of ion flow.