Same reason chainstays are ovalised in the direction they take the most stress.
Except they are not. Ovalised stays have the major axis vertical, whereas the greater stress is lateral, so it would be better to turn them through 90° if stiffness were the only criterion.
Except they are not. Ovalised stays have the major axis vertical, whereas the greater stress is lateral, so it would be better to turn them through 90° if stiffness were the only criterion.