Wouldn't a modernist conception of history also be a sacred belief? Historians haven't worked this way for the better part of a generation now.
No, because, again, according to Haidt, universities have become ideologically stratified along progressive lines. Where even a decade or so ago there the ratio was one conservative to five liberals it's now more like 20 liberals to 1 conservative, and most of the right wing types are in the hard sciences, business, etc. The liberal arts have very little conservatives. Haidt was doing an interview and said to the interviewer that in his field there is one single conservative. Period. He said people laugh when he says that, but no one has come up with a second.
And if you have ideological homogeneity you have no one questioning the ideologically born constructions of other academics.
No one questions, on the Right, that by our standards people from previous generations were grossly unenlightened, racist and unsophisticated (not just here by everywhere in the world) and also much more prone to violence. But aside from progressives we simply accept them with their warts. For progressives, their racism, sexism, and lack of inclusiveness seems to produce hardened attitudes of angry disapproval, which then rejects everything good they did.