The Native Americans were unhappy with Oklahoma becoming a territory because major parts of Oklahoma had previously been set aside as "Indian Territory." Therefore, when Oklahoma became a state the Native Americans faced greater pressure to give up lands from new white settlers from the eastern states.
In addition to the previous answer, the Natives' fears were coming true: the white man had finally won. They were being forced to live with many other tribes on very small, unhealthy reservations. President Jackson forced them over the Trail of Tears into their reservations.
