Question

why AC current is used in transmitting power over long distance

Answer 1

 AC cable would have much higher capacitive and inductive losses and so high voltage DC cables are always used for long distance power transmission.

Answer 2

AC current is not used for transmitting power over long distance transmission.
actually HVDC(high voltage direct current ) is used for transmitting the power over very long distances

AC is though used for transmission coz it is easy and cheap to transform one voltage to another using transformer but these days HVDC is more preferred.