The actual reason is that a boot of lightning comes down from the sky. Watch the animation. Its a vertical attack. I was amazed the first couple times I got killed sky high above a titan, then I payed attention. It has a column. You've got to jump to the side of it or you're toast. But it isn't as if a bladeprancer can't melee someone 30 ft in the air either so....voidwalkers are really the only ones it's effective to get above.