[aparks517]

Power is the product of current and voltage. To transfer much power at a low voltage requires a large current. Supporting a large current without getting hot and melting requires large-diameter wire. Other components may also be bulky or expensive if they have high current-handling requirements.

I seem to recall that somewhere around 50 volts is a magic number from a safety/regulatory perspective. It might also have something to do with semiconductor breakdown voltage, but maybe I'm out of date on that.