Likely because of the alternative. No democrat who’s run against Trump has embodied anything resembling someone who holds Christian beliefs. Trump has faults (and there are many) but I don’t think he’s advocated for domestic abuse or adultery. Biden and Clinton on the other hand openly stump for abortion, gay marriage, transgendering children, etc. The Democrat party as a whole derides people of devout faith. Calls them rubes and uneducated. They are a basket of deplorables. Clinging to their guns and bibles. Why would you think that group of people would vote for candidates who feel that way about them?