why

At high a.c.  , transmission is done at low current , thus energy loss in the line wires due to heating ( directly proportinal to I²R ) will be drastically reduced i.e., the transmission losses will be low. Further , when the current is low, we can use thinner wire for transmission and thus we can save a lot on the cost of metal used for wire and huge support needed for laying the wires overhead to a long distance. So electrical power is generally transmitted over long distances at high a.c.