Because religious fundamentalism is the least moral, faithful, truthful religion.
In the US, fundamentalism is additionally shot through with what's known as "dominionism" or "Christian reconstructionism", which seeks to push us toward a theocratic governance, and in its purest form, to even supplant our legal system, based on British Common Law, with Old Testament religious law. Hence they seek worldly power as an end in itself, to be ruthlessly obtained in the name of god and in the interest of piety.
He ran under the Republican ticket, and they have been told Republicans are the Christian party. I would say a very large majority of people who vote don't bother to look at the candidates, their proposed platforms, or their history of prior government service. Thinking is too hard, so much easier to be naive and vote as your told. After all, God is in control, so no need for me to spend the energy trying to make things better.
The only answer I can come up with are his appointments to the Supreme Court support their views.
That's the proximal motivation but seizing ideological sway over the courts at all levels is just a prelude to what's to come -- the recasting of our entire legal and judicial system along Old Testament lines.
If you want to see what that looks like, make sure there are no guns, sharp objects or long ropes in your house, and binge-watch The Handmaid's Tale. The details will be different (we don't currently have a fertility crisis, but some sufficiently threatening perceived crisis will be used to justify such a regime). But the general concept is scarily spot-on, particularly if, like me, you come out of fundamentalism originally.
There's a lot of ways for this to play out and many of them don't lead to a Christian Taliban scenario like that. But a lot of them do, and they're now dangerously plausible, thanks to what Trump has normalized. And in supporting Trump, Roy Moore, and similar perverts for high office, the Christian Right proves that it's ready to go all the way for Jesus.