For what reason might it be that oral contraceptive use decrease ovarian cancer only if the women isn't menopausal?

The hypothesis is: that contraceptive hormones which block ovulation of reproductive age women seem (in epidemiological studies) to offer a reduced risk of certain types of ovarian cancers because they prevent the recurrent trauma/inflammation of follicular ruptures at ovulation. Since menopausal women no longer ovulate, these hormones are no longer protective.