The reason for lactose intolerance is the lack of production of a certain enzyme, lactase, which aids in the digestion of milk and other dairy products containing lactose. The point of origin is in Europe, particularly southern Europe in Italy, where lactose intolerance seems to have points of origination. Populations that have consumed milk and milk products regularly have a tolerance for lactose, well past the weaning years, on into adulthood. Basically, if you consume milk and products that contain dairy products on a regular basis, as part of your standard diet, the body continues to produce lactase, which aids in the digestion of these products. Where milk and dairy is not a staple consumable product, the production of lactase diminishes, and can completely disappear, producing a population of adults who are lactose intolerant.
Populations that are coined "pastoral", who reside in rural settings, depend more on milk and dairy products. Populations who reside in metropolitan areas, cities, depend on what is available as a food source. It is conceivable some cities do not have as dependable access to fresh milk and dairy products as others. This might be a factor causing the drift across continents of lactose intolerance.