To put it bluntly, it was history about white people for white people. All this changed, beginning in the 1960s. The civil rights movement gave voices to blacks and Indians, and changes in immigration laws brought a massive influx of non-whites. It was the end of a certain kind of America. Non-whites began to complain about a version of history that left them out.
History has winners and losers, and they see the same events with different eyes.
The old history called it a triumphant advance for civilization. But for Indians, the same historical events are an unending sequence of defeats and disaster. At the time, it was thought a glorious success because it added huge chunks to the American West. Slavery poses a similar riddle. Blacks want to make it the centerpiece of their history, and in many ways it is.