Because of flower children and the Vietnam war.

Ok, that's a bad joke. The rise of the perspective of native peoples in U.S. historiography came at a time when the events in Vietnam and the civil rights movement were forcing many Americans for reconsider the previously-established preconceptions they held about the innate moral value of non-white persons. Additionally, the strategies pioneered by the civil rights movement inspired a variety of Native American groups, such as the American Indian Movement, which led protests actions and other campaigns that led to a greater awareness among educated liberals of the indignities and injustices that had been afflicted on Natives over the last few hundred years. One of the most well-known of these was when the AIM siezed the replica Mayflower on Thanksgiving Day of 1970. This event was widely covered in the media of the time, is symptomatic of the larger shift in American conceptions of race and identity that was going on at the time. All of this is gleaned from a documentary I saw a few years ago about the AIM, entitled something like "Taking AIM: the story of the american indian movement".