Because the British had shown to be less agressive in their expansion efforts, even going as far as prohibiting American Colonists from colonizing territories of the Ohio country, which the British considered to be American Indian territory.
The American colonists on the other hand, were more aggressive because most of them were poor and needed land to grow and survive. This is why during Colonial America, they pushed further and further west into Native American territory, causing frequent conflicts with the Native Populations.
