Isn't it just the logical consequence of non-European immigration? AFAIK lactose intolerance is a pretty well-known phenomenon (unlike gluten): mamals are lactose intolerant after weaning, and as such most humans are so. There exist a few places (northern Europe, subsaharian Africa, southern? India, somewhere in the middle East) where the same kind of nonsense mutation on the gene responsible for weaning appeared, so people from there are usually lactose tolerant even after being weaned. The more people not coming from there there is in your population, the less lactose tolerant your population will be.

Edit: incorrect english wording