How come only jews get tay sachs?

Heredity: Jews were isolated in communities in eastern europe when they were forbidden to mix with the rest of the populace. At some time about 300-400 years ago, a mutation occurred which led to tay sachs in that community. Since outmixture was not common, the gene stayed in that population and since it was recessive, it could propogate. Jews of that origin can marry and the disease shows up.

Misconception: Alhough Tay Sachs is prevalent in Ashkenazic (Eastern European) Jews, the disease is also found commonly in French-Canadian, Old Order Amish, and Cajun populations. These are relatively insular, isolated communities in which Tay-Sachs mutations occurred and then propagated. https://ghr.nlm.nih.gov/condition/tay-sachs-disease#statistics