Why are Alzheimer's rates higher in developed countries?

I recently looked at a map of the disability-adjusted life year rates from Alzheimer's and other dementia-related diseases. I was shocked to see that developed nations like America and many countries in Western Europe had very high rates of Alzheimer's. Is there a reason for this? I would ume it's because of the long life-spans here, but I would have thought that proper diets and accessibility to medicine would counteract it. Thanks for any answers!