Not just Christians…
Fundamentalists, whether Christian, Hindu, Moslem, Jewish, or Buddhist, are conservative, in some cases even reactionary. Why? ¹
Glad you asked.
Fundamentalists are certain beyond any possibility of debate that their preferred sacred book(s), inerrantly portrays God’s, (or the gods’) will, which is to be followed to the letter.
Because they’re so sure they’re absolutely right, its simple logic that they regard any differing positions as absolutely wrong.
Therefore, though it may not be legally justifiable, the more militant² fundamentalists feel morally justified in imposing God’s, (the gods’) will on everyone else. After all, God’s, (the gods’) will, perfectly expressed in (each group’s preferred) scripture, must not be defied.
Social change is strongly resisted if it doesn’t square with ancient patterns of social organization reflected in their scripture(s). What was condemned, say, 3000 years ago in scripture is to be condemned forever, because (each group’s preferred) scripture is a once-and-done-for-all-time perfect reflection of God’s (the god’s) eternal will—hence the social conservatism or outright reactionary tendency.
¹ Fundamentalism crosses religious lines. Therefore discussion in the round is tricky because the subject is inherently complex. This post, a first effort, may change to reflect evolving understanding.
² Some fundamentalists are content to “stand on (their preferred) scriptural truth and love the sinner.” Unlike the more militant strains, they live law-abiding lives. They don’t riot, shoot people, bomb crowded pizza parlors, or otherwise shred public peace, dignity, or safety. Neither do they conspire to do away with democratic traditions or institutions, the better to impose rigid theocracy on everyone. Theirs is the lawful activism of the ballot box. In short, their intolerance is tolerable.