Capacitance is a major reason; the capacitance of the very large line doesn't need to be charged and discharged at very high voltage 100 or 120 times a second (

Capacitance is a major reason; the capacitance of the very large line doesn't need to be charged and discharged at very high voltage 100 or 120 times a second (per half wave of 50 or 60Hz). Also, an X volts RMS AC line is 2.8X volts peak-to-peak. The peak voltage is limiting with regards to breakdown voltages, and so DC can reach a higher equivalent voltage on a line with the same maximum voltage.