Even by the 1860s the Great Plains were not settled. Regarded as the Desert, White Americans had settled around it in the East and West.
This Homestead Act gave 160 acres of the Great Plains in return for a nominal fee in order to encourage the settlement of the last piece of the USA. The Government was keen to control the whole of the USA, and part of the Manifest Destiny the Plains could not be left to the Indians, it must be settled and organised.