Because a single payer would, by definition, cover everyone.

Private insurance companies and employers (the two places most Americans get their health insurance from) may not want to pay for the higher costs associated with higher risk people like this. Employers would be especially susceptible to this sort of discrimination if proposed legislation allowing employers to demand genetic tests of their employees passes.

https://www.washingtonpost.com/news/to-your-health/wp/2017/0...