Why is electroplating always done with direct current rather than with alternating current?

Because you want the deposited metal to stick onto the base metal that you are electroplating, i.e. the ions flow from one electrode to the other. and for that, the flow of the current has to be in a single direction, as it is in DC. in AC, the flow alternates periodically, and you will not have any deposition at all as the ions may just go back and forth.