Why did the policy of treating the great plains as a huge reservation change?

Policy of treating Great Plains as a huge reservation changes because white settlers began wanting land on the plains.  

Explanation:

Settlers are the one who migrate from the other area to that new one. Jamestown, Virginia was the place where first colony was established and many people settle there as they were persecuted by various regions in 1607. In '1770', more than '2 million' people settled to 13 northern American colonies. Because of more people settling there, reservation policy changed. 

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