They get manipulated about and distracted by certain issues. The people who want power know this and exploit topics such as guns, abortion, fear of crime, racism/nationalism, sexism, economic issues and taxes. Plenty of people vote republican because they have been convinced that Democrats will take their guns, allow in too many immigrants (with the implicit idea that immigrants are bad somehow), be worse on the economy, lower taxes, let criminals get off easy, reduce the influence of Christianity, and so forth.

There’s also the decades of propaganda about socialism and communism, and against social safety nets as well as government and anything run by the government vs a private entity. So basically, because they’re not very aware or well informed and all themselves to be convinced by propaganda.