Why is it electrical power is normally transmitted at high voltage?

Simplistically Power is Voltage times Current. Sending at domestic voltage requires much more current to be sent for the same power and therefore much thicker wires, heavier equipment and more repeater Transformers as over distance the resistance of the wires and switches would cause the voltage to fall quickly below domestic. For the same power delivery, sending at higher voltages requires less current, lighter wires, and voltage losses over distance are more easily managed so much less equipment is required.