I recall reading in numerous places that the reason for that is so that the same bus fleet can be re-used to bring the elementary school children in at the far more reasonable time of 9 AM. I don't know if it's true, but if it is it's a uniquely American social phenomenon - inadequate public transport, a strange mixture of nanny state mentality and indifference to the welfare of children, and penny pinching all combining into a perfect storm of chronic sleep deprivation for an entire demographic of developing brains.
Another explanation I've read is to make room in the afternoon schedule for varsity sports practice. Honestly I'm not sure which is worse.