answer:A lot of the states that have them were founded from the midpoint to the end of the U.S.‘s history. During that time, there was a lot of concern over ensuring healthy families and, unfortunately, a distinct eugenecist thread to it all (race mixing would lead to inferior races, etc.). So…much of that ban was probably in place in order to ensure that inbreeding wouldn’t yield unsuitable child citizens (heredity study was fairly new), and was kept because no one is really stomping their feet in court to get that ban dropped.