It has been explained to me once in more details than I care (to remember), but the short answer is, it has to do with regulations requiring the body to handle 16G and also to prevent the passenger's head from smashing into the "wall" of the suite directly ahead, in case of steep deceleration.

If you think about it, in a straight axis-of-forward-motion line, there isn't that much of a distance between a hard object and the passenger head in a herringbone seating arrangement.