The Rh- cells in a mother attack the Rh+ baby and kill it. That is why mom's who are Rh- have to get a rhoGAM shot after they have a baby, so if the next kid is Rh+, the mom's Rh- blood doesn't kill it.
Rh- is a freak of nature and does nothing to sustain procreation. Why does it exist? Where did it come from?