Why Ac is more benefitial than Dc for long distance?

Very high voltage is preferred for long distance transmission to reduce the copper losses. Copper loss = (I^2)*R Power = √3*V*I*pf. For the same amount of power, as the voltage increases the current decreases.In other words, the same amount of power can be transmitted by stepping up the voltage to HT or EHT by reducing the current considerably and thus minimize the transmission losses. Also if the current is small transmission conductor size can be reduced considerably. In ac stepping up and stepping down of Voltage is very easy. But in case of dc it is not possible. So for long transmission ac is preferred.

only one advantage , we using transformer for increasing the travelling voltage & decreasing the current to minimize electrical loss.and we can not using transformer in DC voltage as there is no frequency 

Because we can stepup to AC supply easily