Was it in part due to the civil rights movement/AIM, international publicity, the end of the cold war/COINTELPRO targeting of narratives, and the rise of the information age?
Or was it due to something else?
(Hint: AIM, IITC, 1977 Geneva Convention. Cross reference membership with literary output in subsequent years. Cross reference literary/flier content at colleges/state events on Indigenous Peoples Day with key words/phrases and associated authors.)
Greater willingness of scholars to approach history from multiple perspectives, mostly.
It tracks along with similar changes in the way that other well-known and highly mythologized historical figures are being re-examined. Jefferson, Washington...
Historians today are increasingly interested in producing more nuanced-- and contextually accurate-- depictions of historical figures and events. This comes in part from increased sophistication of analysis and availability of data / information, but also from a growing sense that history told from one perspective-- i.e., Western civilization, or focusing on only historically important individuals rather than examining the social, economic, and cultural conditions and circumstances in which historical events took place-- tends to distort the broader picture.
The recognition among historians that historical narrative has often been a tool of imperialism and colonialism has also played a role. And that comes from the broader awareness of the world and other cultures and histories that-- increasingly-- global communication and interaction has enabled.