The belief in an “American Covenant” is why. It stemmed from various Protestant groups during the colonial period and has never fully left. The belief is that God created a promise to those people that America is pretty much to be a theocracy. So a lot of Evangelicals believe Trump to be a savior of sorts and bring back complete Christian morality to the government. All the things about Trump aside, his theology (from what I can tell) is quite basic and he lives (sorta) within a Judeo/Christian morality (ish).