I would suspect, that if you are looking at per-capita consumption, a large factor would simply be that the proportion of the population that is adapted to cons

I would suspect, that if you are looking at per-capita consumption, a large factor would simply be that the proportion of the population that is adapted to consume dairy products has shrunk. Drinking milk is, in some ways, kind of a weird accident, and the genes responsible for making that possible are clustered in a few different regions. An increasing share of immigration and population growth in the intervening decades has come from populations where those genes are rarer.