I'm going to do my best for this, I'm not sure about Canadian history, so I'm not sure how applicable this will be to Canada, however I'm sure there will be some overlap, I really hope this is coherent, if it's not please inform me so I can try to fix it

Firstly, native Americans in the east have been in contact with Europeans for far longer, subjecting them to devastating European diseases far sooner than tribes in the west, as well as wars with European settlers much earlier than the western tribes, and although places like Nevada, California, and the Four Corner States, were officially under Spanish and later Mexican control, these areas were largely left unpopulated by Europeans, with the only notable exceptions being Catholic Missions.

Another important thing to consider is the relocations of many tribes under the Indian Removal act, the most famous of which was the Trail of Tears, in which many Native tribes in the south east, famously consisting of the Cherokee (Georgia), Creek (Alabama), Choctaw (Alabama) , Chickasaw (Tennessee) and Seminole (oddly enough the Seminole were in Spanish owned Florida) were deported from their lands in the east, into modern day Oklahoma.

While western tribes would go through very brutal wars and bouts of disease, and they would also be relocated, they, usually, were not moved across the country, unlike the eastern tribes, instead the majority of their land would be annexed by the United States, and small reservations would be established on a very small section within lands that had previously owned by them.

TL;DR: although both eastern and western tribes had endured atrocities committed against them as well as devastating bouts of disease, the eastern tribes were exposed to this treatment far sooner, and we're later deported to the western parts of the US