Suppose class mobility is good for poorer people. Canada has more class mobility than the US. Canada also has universal healthcare (like most developed countries) which helps people take risks like starting their own business which helps with class mobility.

Why do poor republicans in the US keep voting against universal healthcare?

They actually don't. You get that result only if you exclude non white people.

Overall, if you define working class or poor economically, they are more likely to vote for those things.