One Hypothesis:

We think that the reason some races have a higher percentage of lactose intolerance is because some people drink more milk due to their cultural foods. For instance white Americans maybe have a lower percentage of lactose intolerance because a lot of the native foods include cheeses, and a lot of items with milk, as compared to Asians who have a high percentage of lactose intolerance because many of the natives foods do not include milk, so they are not accustomed to eating it.