The answer is: Because the British promised to the Native Americans that they would control the expansion into Indian land.
European colonization of the Americas has led to centuries of conflict in Old and New World societies. Most of the written historical record of Native Americans was made by Europeans after initial contact. Native Americans lived in subsistence hunter / farmer societies with value systems significantly different from European settlers. Cultural differences between Native Americans, on the one hand, and Europeans and their descendants, on the other, led to large and lasting war and cultural conflicts.
