Genetic reasons are not favored, at least according to what the author of the article, Pam Belluck has gleaned:
"It is not that Hispanics are more genetically predisposed to Alzheimer’s, say experts, who say the diversity of ethnicities that make up Hispanics or Latinos make a genetic explanation unlikely."...and what this researcher has found:
Dr. Rafael A. Lantigua, a professor of clinical medicine at Columbia University Medical School, said, “There’s no gene at this point that we can say this is just for Latinos.” Dr. Lantigua added that one gene that increased Alzheimer’s risk was less prevalent in Latinos than non-Hispanic whites.More favored explanations are SES, "cultural dislocation", and depression.
The author goes on to describe several studies that have examined the etiology of population differences in Alzheimer's, one of them finding that higher acculturation to American society among Mexican Americans was associated with higher Alzheimer's risk.