It has to do with the fact that children are not lactose intolerant... lactose tolerance is born into most species and then tapers off as the child matures into an adult. The age at which it tapers can vary from person to person but generally if the child is born lactose intolerant then they cannot breastfeed and since that is the natural foodsource, those children would have been naturally selected for starvation until modern times.
BTW, no one said 80%. I said 50%. You're also only talking about one isolated area where there is likely a lot of historic racial mixing, skewing the racial dataset.
50% is worldwide, which if you went to Africa, where there is far less racial mixing, the stats go way up and that's how you get your worldwide average.
I have a close black friend from that area of Texas and just asked him. He says that a lot of folks there actually are lactose intolerant but have no idea why they have problems. He also says the school systems pretty much gives the milk to the kids with no alternatives other than being hungry. This would really make a difference in a poor area where that is probably the only real meal a kid gets all day.