The reasons Native American culture was irrevocably changed by the end of the 1800s.

Western migration was supported heavily supported which meant that people were taking the Native land more and more and forcing them into reservations. The natives who wanted to become Americanized would get great benefits from the government so them abandoning their culture was a common thing because of all the things they'd get like land and similar things. There was literally nothing that supported their individual rights which caused many to abandon their culture and those who didn't want to suffered problems, from punishment to full on wars.