I see two reasons. First a history retrospective effect. Then, the same schedule of primaries, focusing on states where evangelicalism dominates: 60% of Republican voters in the Iowa caucuses, 55% of white voters and Republicans in South Carolina. The palm of the hyper-evangelism back to the former Baptist pastor Mike Huckabee, but even the Mormon Mitt Romney spoke of Jesus, his savior. Barack Obama spoke of his conversion, and Hillary Clinton prayed with surprising frequency….
Yes, in the camp of religious evangelicals of the Republican right pure and simple, short the idea that the American nation is a Christian nation and it is time to correct the Constitution omits any reference to God. The Christian right can not admit that American institutions are a creation of the Enlightenment, inspired by the wicked, agnostics or deists like Thomas Jefferson. The latter was accused of atheism in the presidential election of 1800 (he won). They said he wanted to throw thousands of Bibles to the stake and install, as Jacobin France, a worship of the goddess Reason. Jefferson was not a Jacobin, but it was Voltaire.
However, the First Amendment prohibits the federal official church, but did not comment on the local or federal. Some states like Connecticut and Massachusetts will maintain official churches until 1833. These little theocracies finally disappear under the weight of public opinion.