Why were Native Americans forced to abandon their Iand and move west?
American's unquenchable thirst for the land upon which Native Americans lived.
The first example of this happening was in George where the Cherokee were forced from their lands which became knows as "The Trail of Tears." That was in 1830.
After the Civil War Americans started looking at Oklahoma to expand into. The civil population wanted to settle on Native American lands, the government wanted it and to insure it thousands of cavalry were sent to protect the settlers.
Strangely, in 1868 the US Government made a treaty with the Sioux Nation that it would not expand any further into their lands. But in that same year the transcontinental railroad was begun and the government was well aware that more Native lands would be taken.