Because you'd need thicker wire to carry the same load at half the volts.
ok, so lets go with this. lets say you have a 1kw hps. you use a 20a 120v circuit. that'll work, and you'll have 1400w still availvable.
if you use a 10a 240v circuit, you can use the same wire, and still be able to run an additional 1400w.
however, if you double or triple the current, you need to use a larger wire to prevent a fire. watts = lost energy = heat.
so, a 20a 120v circuit will run on a 14/2 gauge wire.
a 20a 240v circuit will need TWICE the wire as it is supplying twice the power. (10/3 for a dryer at 30a, and that's code) even your electric range runs off 10/2, as does your electric water heater. big power, big wire....
it's not rocket surgery
i'm not picking on you, a lot of people have a hard time comprehending this in college. i'm an electrician, and i am NOT lying to you.