why is the high Voltage Transmit ?

When the current is transmitting through the cables,power may be loss due to heat.
that is you can understand by using below equation;

power(P)= I*I*R


I= current through cable(Amp).
R= Cable Resistance(ohm).
P= Power (Watt)
When the current is increased, Power may be increased according to the above equation.therefore power loss is increase.

in order to reduced the power loss, you have to reduced the current through the cable.
According to the Ohms law, to reduced the current you must increase the voltage . as below,
V=I*R

therefor High voltage used to transmit the current.