We are losing the actual factual basis of our common history for many reasons, but a big factor is because people in charge of the educational process don't want to talk about things that bother them, don't want to include things they disagree with and want to include things they do agree with that have marginal if not counterproductive utility.
While I'm not entirely clear on how he uses the term "political correctness," and he dismisses hip-hop as a tremendous cultural force, Bob Barr's examination of the purpose of history text books in public schools is spot on. And yes, he is messin' with Texas.
I'll not dilute his words by quoting. Go read the whole thing.