Because they're using a single nut instead of 5 and the larger the nut the more likely it is to get rotational input from the wheel?
You don't need to reverse the direction if you've got a multiple bolt pattern with relatively small diameter lugs. Which is why millions and millions of cars with standard lugs don't have wheels falling off constantly.
Correct, PLUS the bevel in the wheel and on the 5 individual lug nuts locks the nuts in place. If I am not mistaken, the single large nut they use in the center has a flat contact surface. So they need to be directional in order not to back off.
