Basically, the higher the voltage the less current is required for a particular power.
Therefore the lower the current, the smaller the cable needs to be, reducing material costs.
However, the higher voltages require extra insulation and precautions, where it is able to be accessed.
When I was serving my apprenticeship as electrician, way back in the 60's, there was rumour of a 1MV transmission line.
Poly-phase systems, in particular 3 phase, are used again for the same reason.
3 Phase power (in watts) = Sq Rt 3 (1.732) x line volts x line current x power factor x efficiency.