They want to tear down all historical narratives but their own so that no one can oppose what they see as the logical and unstoppable spread of progressivism.
These social justice warriors are like the ISIS members who entered museums to smash millennium old artworks because it was contrary to Islam. It was an affront to Allah to have anything that indicated civilization existed prior to Islam.
Great comparison. Progs & Isis as enemies of civilization.
It is an interesting feature of totalitarian dictators (or wannabes) that they must always erase history as part of subjugating the populace. If their way was really better, they wouldn't need to erase history--it would be self-evident.