In my opinion, this, my friends, is a classic example of the progressive philosophy at work. I say this a-politically. Let me explain.
The primary premise of the progressive philosophy, as an outlook in general, is, "Things and people are getting better." That is the fundamental thought underpinning the outlook. It is sacred and central. History is a hill that we are pushing mankind up. We must take the steps to see to it that the future is better than the past. The present is, and must be, better than the past. The problem is, when carried to its extreme, this thought process simply doesn't conform to reality. There is good and bad in most ages of man. So, what do you do when confronted with things or people in the past who aren't worse than, and perhaps are better than, those of the present, and thus don't conform to your foundational idea that everything today is better than it was in the past? Unless you are really honest, you begin trying to find ways to tear down those things of the past so that they can be worse than what we have today and prove your philosophy.
And this is what you see repeatedly in progressive historians: a philosophically-driven need to do whatever is necessary to drive history forward and up, even if it means shoving the past down.
Bob