This is a way oversimplification and just my opinion, but I think it boils down to 2 things.
1) The two-party system inevitably creates an "us" vs "them" mentality. It's just human nature. There's no room for compromising an alliances. Your side either wins or loses. And nobody wants to lose. So even if you're not thrilled about your candidate, you're more likely to vote for them because there is no other option
2) Conservatives in the US in general tend to be single issue voters more so then Liberals. So, they pick one thing that they care about and then vote for whoever best fits that policy. Unfortunately in many conservative pockets that tends to be an anti-abortion, or anti-immigration, or pro-gun, or pro-blue collar jobs vote. Which the Republicans frame their whole campaign around. And it works
It's obviously much more complicated then I can ever hope to fully understand or explain but in my opinion those are the 2 biggest reasons he gets that support.