In the 2018 poll the majority of Republicans (52%) actually supported it. There's just been a lot of effort pushing against it by most of the leadership in America (the media, the Republican party, most of the Democratic party leadership). The media plays a big role in this - they often only focus on the cost of one particular implementation of single-payer, and not on the 10's of thousands who die each year due to lack of insurance. There's no mention that the U.S. government is currently spending more on health insurance than the U.K. government spends on its single-payer system (and the total government and private spending in the U.S. is much, much higher).

And even though Democrat voters support this the most, the majority of Democratic Presidential candidates this year were opposed to single-payer and pushed some attacks on it that were mostly false (arguing that it removes choice). And a lot of polls on it will mention things many Americans view as negative (a single government run system) and not mention ones that they would view as positive (no out of network doctors, never being uninsured, never switching insurance, no medical billing issues, no healthcare bankruptcies, lower healthcare expenditures, etc.).

Have you seen the federal government lately? It's totally understandable not to want that to be in charge of your healthcare.