If current is high then I2R losses will be more .If I2R losses is more then heat generated is more. By heat or increasing temperature resistance will also be increase which will decrease transmission efficiency.
Also we know P = V x I
where, P = Power,
V = voltage,
I = current.
By changing the value of voltage and current we can get constant power. By increase the voltage we are decreasing current which is advantageous. That's why transmission voltages is high.