Two reasons:
Howard Zinn, and Bill Ayers
I think he’s retired now but for a couple of decades Bill Ayers was THE LEADING and most influential academic in the U.S. when it came to formulating curriculum.
I know. How could we let THAT happen?
Howard Zinn basically perverted the teaching of history. I was a history major when his book came out. Even though I was a young skull full o’ mush I was disgusted by the multiple-orgasms they were having in the faculty lounge over his work.
I still can't comprehend how a known domestic terrorist became a figure of respect in academic circles. I blame this on being a rational human being.
And as for Zinn, how in the world did a "history" book with no footnotes or endnotes to back up its claims become so legendary, other than because it was mentioned in Good Will Hunting?