I'll try to explain it. Hope you will get some idea & reasons. Before getting an idea, please have a look.
Electricity Flow: AC vs. DC
Electricity flows in two ways: either in an alternating current (AC) or in a direct current (DC). Electricity or 'current' is nothing more than moving electrons along a conductor, like a wire, that have been harnessed for energy. Therefore, the difference between AC and DC has to do with the direction in which the electrons flow. In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forward" and then going "backward" from positive to negative voltage.
With DC voltage, there is steady voltage available which in AC voltage has to be converted. The breaking capacity in DC is considered lower than the one in AC.
Regards,
Syed Yousuf Ali